The general answer is that the park hopping feature applies only to the 4 major parks. The water parks fall under the "plus" option. With the new ticket struture, you really can do what every you want.
This is what I believe price options are for you
5 day advance adult ticket expires / no expiration
Park hopper/no plus $235 / $273
Park hopper/ 3 plus options $283 / $320
Single day water park $36
If the water park is the only only plus option you want, I would probably stick to the hopper with no plus options and buy single day tix for the water park. Unless, of course, you want a 5 day hopper that does not expire - then add the plus option since the plus options will not expire either.