The benefits of having the new Hero 3+ are:-
Better battery life, approx 20-30% better
SuperView, extra wide angle
Faster wifi, less lag, around 1sec now compared to around 4secs
Smaller housing, smaller and lighter but less waterproof, only 40mtrs compared to 60mtrs. Although you can still use the Hero 3 housing if you need the extra 20mtrs
There have been some complaints of the Hero 3+ have less focus at distant but it looks like this could be resolved in the latest batch of Hero 3+ cameras.
The biggest benefit for watersports use is the extra battery life. Changing batteries underwater is impossible and changing batteries around water isn't a good idea.
So for me, I think I'd recommend the 3+ purely for this reason.
Having said that, if you wanted to fit a battery bacpac to the Hero 3, that would give you better battery life, but is an optional extra.