Our vacation was spent mostly in water. You had many vessels to choose from (speed boat, fishing boat, paddle boat, pontoon boat, kayak, wave runner)...activities ranged from talking on the dock, swimming (of course), tubing, skiing, jumping on the Rave (a big floating trampoline), playing King of the Rave, jumping from a crudely constructed rope swing, shooting water at the 'dry boat', playing with the kids in the shallow water, floating and chatting in the deep water, sand play, fishing....
Last family reunion I was extremely pregnant so swimming was all I did...but this year, water skiing was definitely on the agenda. Everyone else made it look soooo easy, a 5yo did it! Our 11yo had lots of trouble (so did I)...but after some nice face-plants...I did get up but it didn't last long (but the soreness in my hamstrings did).
I also really enjoyed kayaking (shhh, don't tell my hub, he's been wanted a kayak for many years). It was really relaxing and felt easy. I also liked being able to talk with my SIL while we tried to paddle straight.
It was really nice spending the days in our bathing suits playing around. We really had a hard time saying to was really time to leave (one more boat ride? one more swim?).