Other people on the boat are happy to take pictures for you. Also take a disposable camera up there to capture great shots of the ship and the island. You can also go onto Deck 10 of the ship and get some pictures, but you won't know for sure if it's your family member, or some stranger since you're not close enough to see them. I did this, and was sure it was my DH. When we got the pictures back, he said "I didn't have the smiley face parachute, I had the xxxx one". Oops!
Banana boat would be harder to do pictures. The trip is pretty rough, so it's a fair guess you would lose the camera. And there's water flying everywhere so you won't be able to see very clearly. Another member of your party can come down to the loading area with you and take pictures as you're getting on the banana boat. But once you pull out, again, no one could tell if it was you or a stranger.
I didn't see anyone professionally taking pictures of these activities. It was one of the few places there wasn't a photographer!