DHS needs a greater number of variety of ride. I don't think any particular type of ride is needed. I don't really see the "need" for a boat ride like POTC or IASW over any other type of dark ride mechanism (the park could use more dark rides, mind you, but there's no reason a boat ride would be more beneficial than, say, an omnimover).
OTOH, if people are suggesting the park would benefit form a water ride that (intentionally -- sorry MK's POTC) gets you wet for the cooling effect, I think there is an argument there to be made. A park doesn't have to have such a ride, but given the hot Florida sun, they are often popular and certainly would diversify the offerings. Then again, Epcot doesn't have any rides like that either.