You've been given great advice by fellow UHHers. One suggestion, you mentioned doing posed shots of bride & groom. Could you do those say the day before when everyone is more relaxed without all the family & friends rushing about wanting to take their own cell photos of the couple being posed by you? These would be photos of just the bride & groom. Then on their spcial day, you can shoot photos of them with family and friends. Nothing worse that trying to get posed photos with other people taking cell phone photos, getting in your shot or their flashes going off just when your shutter goes off and ruining the shot. You mentioned possibly using reflectors - so you will have someone to assist you by holding reflectors - so you will have an assistant then. 2nd camera with other lens is good idea since you said there were other activities like jet skiing which would require telephoto lens, if you can manage it. Don't forget getting photos of bride & groom getting ready. We hired a professional photographer for our casual wedding reception in our home, as we got married at wedding chapel who had their own photographer - he shot casual photos of guests, cake cutting, etc. and his assistant shot photos of rings, cake, flowers separate from him. We were very surprised and pleased with some of the assistant shots that we didn't know were taken. Guess that was more than one suggestion. :-) Just remember - shoot, shoot, shoot and have extra SD card(s) and batteries. Have a great shoot and try to enjoy wedding also.

Hi,

I think you have the right equipment with the 5D M IV and f/2.8 Canon 70 - 200. I might suggest either a 50 mm f/1.8 or 1.4 or even better might be 24 - 70 f/2.8 II. One other Canon body would provide excellent options. Probably a speed light using bounce features. Perhaps the most important part is to, if possible, to get the venue prior to the event and explore exposure, with and without flash. Once you feel comfortable with your settings, have fun and relax. I'm sure you will provide them with many excellent photo memories. I might suggest you find a way to share the photos after you have culled through and processed with those in attendance - Dropbox, SmugMug, Shutterfly, ...

Enjoy.
Mark

You are most humble & I get the idea that it's not your photography skills in question, but how to shoot a wedding. Don't over think it, just talk it over with the bride as to where to stand & be aware of what is going to happen next. Anticipation is key! By all means use a fill flash especially at that time of day. The trick to shooting events like yours is to anticipate what is going to happen next, and to be in the right position to take the shot. By all means enjoy the after ceremony festivities. Just to let you know, even the most seasoned wedding photographers get stressed. But you will do great. Take lots and lots of photos. I suggest to use your long lens to be far enough away as to be candid.
For your water shots check out his link.https://photographylife.com/lens-filters-explained/
Some will say hire a pro as a wedding gift and enjoy the occasion, but you are a photographer, how else will you learn, & progress. Take the chance and you will benifit .

Formal programmed weddings usually include both enough relatives and friends to keep you shufflling groups into herds, hopefully all enjoying themselves. The informal photos can be a joy to look at many years in the future, with expressions and the awkwardness we all have. Get a list of specially wanted or not wanted photos. Ask when it's not appropriate to take shots with flash or hot lights in use. Inevitably some salient moment will be missed, unless you are a machine gunner. So shoot thoughtfully, but don't spare the memory chip from a workout. It's easier to delete, and impossible to re-create. Hopefully, it's all about love, so make that show.

in preparation for the wedding there are many resources you can check out. Check the photography section at your local library. There almost always books on weddings and portraits that will give you ideas. You Tube has many videos on wedding photography. Also check out Pinterest for examples.

Then meet with the bride and groom and make a list of shots they would like. i.e. the main ceremony, and then shots with parents, bridesmaids, groomsmen, other friends, family groups. Have a second person have the list handy for you. it is a great help to have this in advance.

Good luck with the event.

Hi there! I'm new to the site but have been perusing posts and wanted to send some encouragement your way. I'm not a professional photographer by any means - more of an extreme hobbyist - but have been asked to shoot events, including a few weddings over the years. I always take photos at the reunions and family events but only because I enjoy it; and, I feel like this experience helped me when doing the weddings. Some of the very best shots I capture are candid shots and they always seems to really lend a feel for the mood. People are relaxed and genuine and that comes through in the photos.
You mentioned that this is a casual affair so I would imagine that a lot of your shots will be similar to taking photos at family gatherings. I would suggest getting a shot list from the bride and groom so you know what their "must-have" shots are, and also make sure you have a solid grasp on the schedule so you will be ready to get the absolute must-haves that they want. It wouldn't hurt to have your niece send you a couple of wedding shots that she & her beau-to-be really like (there are thousands upon thousands online via google search or check out her pinterest if she has one!) so you can see what style really catches their eye, too.
After getting all of their must-have shots, you can just drift through the party shooting great moments between fam/friends - there will be a lot of hugs, a lot of laughter, and a lot of happy to capture!
Attending their rehearsal will be super helpful so you can see where they will stand, which direction they will face, etc. Plus, it will be helpful for you to see the house/property where they are getting married beforehand, even if it's the day before, so that you can make a note of any spots that you see that will be great for photos. Be sure to look around and take test shots at the same time that they'll be having the ceremony so you can be ready to deal with the harsh lighting & shadows the day of. Then you'll have a better idea settings-wise of what to expect and can spend more time looking for the photo opportunities vs. the back of the camera.
I would bring both lenses, the flash just in case, & definitely a tripod. If it's tough to travel w/a tripod then maybe another guest wouldn't mind bringing one for you to borrow. As always, bring extra batteries for your flash (I was surprise with how fast my flash burned through batteries at one wedding and was left scrambling to get more), extra camera batteries, and extra memory cards. Oh! A circular polarizer is a must-have of mine and I never shoot anything without one. It will reduce glare and help make the colors pop.
The best part is that if your niece is already a fan of your work then she already likes your style and the photos that you take. They're going to be thrilled to have you be a part of their day.
Apologies for a long (first!) post but I hope that this was helpful and put you a little more at ease and confident that you are in fact the perfect photographer for them.