Hi Greg
Sounds like you've taken on quite a challenge. Your location choice for the shoot is a particularly difficult one. Beaches generally don't have shade, so you'll probably be shooting in bright sunlight. Near noon will put the sun almost overhead, which is the worst case situation. Under those conditions your flash will not really be of much use to you if you don't have the high speed sync capability. A softbox will make the situation even worse, so leave that at home. It takes a very powerful flash to match the sun and the softbox will waste too much of your power.
I faced the same situation with a beach wedding on a destination wedding I did in November in Cancun. Bright sun, bright water, bright sand and no shade. Not an impossible situation, but a challenging one. With the equipment you have, here are my suggestions . . .
1) The better of your two lenses for portraits is the long one (75-300mm) Ideally use it at the widest zoom setting of 75mm (the perfect focal length for portraits on your camera)
2) Leave the flash ON the camera and don't use any diffusers or reflectors on the flash . . . you'll need all the power it can muster.
3) Place the subjects with their backs to the sun. Squinting at bright sun makes terrible pictures and bright sun is too harsh for facial tones.
4) With the camera set on shutter priority and set at 1/200th second, take a few test shots of the background scene that will be behind the subjects. When you get an exposure that gives you a perfect result, change the camera to "M" manual and use the settings that gave the best results in shutter priority mode.
5) Set the ISO at 100, the shutter at 1/200th second and the aperture at whatever worked for the perfect background exposure.
6) I would shoot each pose/subject with a reflector (soft white and the bigger the better) in very close, just out of camera view, and then the same pose/subject with fill flash. If the flash is TTL it will set the exposure needed automatically for the fill. You'll have to get as close as the 75mm will let you.
7) As for poses, google "photo poses" and find some written posing ideas with images . . . download the images and make yourself some small prints that you can carry along on your shoot day as reference. All you really need is a half dozen ideas and that will please your friends to no end. Don't get fancy, but do some reading on some of the basic rules.
8) The above is really for shooting in bright sunlight. In Cancun I had to do that for the wedding ceremony, but when it came time to do the formals, I got them into the shade of some buildings but using the same ocean and palm tree background that were still in bright sunlight. Still exposed for the background and used flash for lighting the subject. The only difference from what I've suggested above is that I had my flashes off-camera.
You can take a look at the results I got at:
http://www.weddingguy.ca/cancun/index.htmlGood luck my friend . . . just keep 'em laughing and they'll love it!