Tina
The 18-300 lens is great, in fact it's my go-to lens but it weighs just about as much as your camera. If yoiu really love the D7100 you might research getting a much lighter lens (not a zoom).

The cheapest case for you D7000 is an Ikelite case plus you'll need a lens housing, total will be around $2000.
If you're only going to snorkel then there are a lot of underwater cameras that good up to 50 ft I would go on line and check out best cameras for snorkeling . I'm a diver and I needed something that was able to go deeper. I got a Sealife DC2000 and my wife got me a SeaDragon3000 lite for my birthday. However if you're only going to snorkel I think that camera might be overkill. Lights are necessary in reduced visibility but you need to have a bracket to keep the light away from the camera to reduce or eliminate backscatter. This photo was taken in Aruba just before the Pandemic.

I use the 18-300 for everything except macro where I use the nikon 105. Its light enough to carry everywhere without strain and provides the flexibility to get almost any image you want. The vr works really well ( I was able to get a really sharp photo of a grizzly bear with the lens at full extension while on a moving bus in Denali park) and I find it very useful for photographing kids from a distance so they don't start posing.

Many thanks for the links and I didn'. I know I've spent lots of time looking for the shot that no one has thought to take. Usually there's a good reason. But sometimes... as in this photo of an old truck along the Chena River in Alaska.And I didn't even have to fight my way to te rail ofthe boat to get the shot

No guess about it . Will pass this photo on to all my friends and to our chapter of Vietnam Vets of America. We can't let what happened to us happen to anyone else in the service.
btw, I was in the 52nd Riverine, skippered a riverboat out of SaDec

I have used the 5.6 version of the 18-300 as a walk-around lens since I got it around five years ago. Its gone with me on a trip through Denali, six weeks through French Polynesia as well as shorter trips and just walking around locally. It's heavy but not as bad as walking around with a lot of lenses. I've found it sharp from 18-300. The picture of the grizzly was taken from a slowly moving bus with the lens fully extended. I was far away -- not sure how far -- shooting at 1/800 second with iso of 400.

https://www.uglyhedgehog.com/compose-post-screen?topicnum=582567&rpostnum=9962556
A lot depends on how deep you want to go and what kind of photos you want to take. I suggest you take a look at the Sealife DC2000. It is more expensive but it does a lot more. It is a 20mp camera with a 1 inch sony sensor. It not only has all the built-in settings you find in any point and shoot but it also has aperture and shutter priority and you caneven set it on manual. It no t only shoots jpegs but also shoots raw photos, a real benefit when you're post-processing. Also it does HD video. I looked at specs for the TG-5 and ruled it out because it is only rated to 50 ft. The Nikon seems a good camera but it seemed a bit limited for what I wanted.
The DC2000 is actually a camera and underwater housing all in one. In the housing the camera is rated to 200 feet. I've never been that deep but I often go below 100. The camera can be removed from its housing and used on land It's nice and light and while it doesn't have the range or versatility of my Nikon 7200 it great to just shove in your jacket pocket when you go for a walk or a drive.
I realize that this maybe more than you want, but if your going to get advanced offshore qualification, you probably will want a camera that can go deep. BTW below 40 feet you usually need a light and the built-in flashes (the DC2500 has one too) are pretty useless
Photo taken at about 65 ft with the DC2500 and attached Seadragon light.

I suggest that you look at the SeaLife DC2000. It has a sony one inch sensor, 20 megapixels, 1.8 lens, takes raw images and videos but it only has a 3x zoom.
I'm a scuba diver and I chose the DC2000 primarily because I can take it down 200 feet whereas the Olympus and a lot of the others are rated for 50 feet or less. But the DC2000 has a lot more to offer. It not only takes RAW but you can set it on aperture or shutter priority or even on manual along with the various settings like portrait or landscape that you can find on most cameras. You can also set you white balance and your ISO . The menus take soem getting used to but are no more complicated than those on most dslrs. Anyway check it out on line and see what you think

Costco has processed all the prints I've sold, from 8x10s to 20x30s. The key is in the quality of your post processing.

How much RAM do you have? You should have at least 8g and even better 16g. I have a rather elderly Asus Zenbook but it does have 16g RAM along with a third generation i7 processor. For photo/video I plug an Asus 28 inch monitor. I run the Adobe pro web suite and have never had any trouble with photoshop or my video and sound processors. I suspect you might need more RAM

Any system is only as good as its weakest (slowest) part. I have an Asus laptop with an I7 running at 2.4 Ghz. It has a 250 Gb SSD but only 8 Gb RAM. Operating system is Windows 7 Pro 64 bit. I bought it in 2012 just before a 3-month trip to Australia because it was the lightest i7 around that was powerful enough for my needs. I've never had problems editing RAW files in photoshop. Six years later and it's still going strong even though it's practically prehistoric in computer terms. One thing I would suggest is an external hard drive for storing photos and anything else that is not a program but takes up lots of room on your internal hard drive. I have two Seagate 4 Tb drives, one I take with me for getting the images off my camera and off my SSD. The other which is really a backup I keep in a safe place.

Aye

I use a vest with multiple pockets to hold my macro and fisheye lenses, my flash, spare batteries and lots of other gear. My camera also attaches to the vest . I've used it hiking over rough terrain in Alaska, volcano park in Hawaii and in the Negev in Israel among other places without getting a sore shoulder or back. I like that everything is in front of me and easy to reach. Don't like things around my neck -- guess i must have been hanged in a previous life

My wife too has a severe back problem. She suggests your wife bring a back pillow and limiting the tour to four hours or less. These are not roads as we know them and can be rough. The same goes for Monument valley tours.

It was very good though banning digital photos from the photo show was a bit much. Think we've seen all the Midsomer series.