When it comes to boating, typically the two most important things are to stay afloat to prevent drowning and to be able to quickly alert rescuers, as extended time spent in the water can lead to hypothermia. Therefore, a PFD (lifejacket) and various signal methods (such as whistles, flares, beacons, marine radios, signal mirrors, ect, ect) are absolutely vital.

With those items, I also consider a knife to be of the utmost importance. There tends to be a lot of various cordage used in, and around, boats. I've personally seen people fishing get tangled up in some line on the boat, fall off the boat, and become so further entangled as to nearly drown. Children especially, are very vulnerable to this. For this purpose I'm a big fan of Spyderco's Salt line, as they use H2 steel which is basically impervious to rust.

Past that, then you're basically looking at a lot of the same gear you would have for land based survival. The big difference though, is that you have to make sure that gear is either suitable for wet environments or specifically protected against against moisture. Otherwise, it might not work when you need it.