There was no way I'd miss this as I'm a sucker for dolphins. I hadn't even planned to do this on my Sentosa day.
I booked the Experience package, I think I paid $97 all up plus $60 for a photo. So not cheap.
The trainer was good but kept getting impatient if you couldn't follow instructions.

Don't recall I visited any one place where my dad smile so brightly and the kid was at awestruck.

Here you spend roughly half an hour playing with a dolphin in a group of 6/7 people. one dolphin will be with your group the whole time, you get to swim, pat, shake hands (fins), and pushed across the pool with them. It was simply amazing.

Couple things to note:
1) There's no need to bring anything expect the swimming suit you put on and yourself. you are not allowed to bring anything (including swimming google and any accessories) into the pool. they have a locker for you before you entry the dolphin area, if you are not planning to stay at the adventure cove you can save yourself some locker money (SDG 10 for a small locker here)

2) You don't really have to be a good swimmer, they give you a diving suit and a life jacket, you can't get cold or drown yourself if you try. you will have to however swim (move) around the pool a little bit, as long as you can do basic swimming and are not afraid of water, you can do this activity no problem.

3) They charge an arm and a leg for the photo. 200SGD for the entry level package. but seeing the BIG grin on my dad and nephew face, I knew I was owned. So the moral is don't come alone, it's better come as a group of 3-6 people IMHO. so the photo cost would make more sense.

4) Book early, I did mine a whole month in advance, they have only limited slot, and I have heard they get filled out quick in summer time.

5) Plan for a visit of at least 2 hrs, you will need time to review photos and you also get to enjoy adventure cove facility.