On longer vacations (7+ days) I can see value and attraction to a water park day(s), especially if with young kids who would really enjoy. But for us our WDW/DL trips were more typically 4-5 days where our focus was on park time. If need be due to heat, we simply went back to resort for one hour or so of rest and pool time, then back to park in evening. But we also have several very good waterparks near where we leave that the kids have been too, and frankly it is true that most young kids remember and consider the resort pool as a highlight of their visit (or MK). Really depends what your party and kids enjoy and want to do. Frankly the whole hot day at a waterpark, and all the carrying of change of clothes, wet bathing suits, towels, sunscreen, and the lack of shade/plus crowds, makes for a long day and for me not really a great relaxing experience in the middle of a WDW vacation. Much easier to walk down from the resort room to the pool for a swim, little sun and then back to room change, and off to park.