Disney offer quite deep discounts to Annual PassHolders (up to 45% off rack rate in some cases) However these are not guarenteed to be available all the time and may only apply to some hotels. An annual pass works out a much better option if you are able to time your trips so that in year 2 you travel two weeks earlier, this lets you get two years worth of trips from one annual pass. If you want water parks to be included you need a premium annual pass which adds $128 to the price. At $34 approx entry per day to the water parks you need to consider whether it is cost effect for you (i.e. how many times will you visit the water parks.)
Good luck
ps Annual passes also give you free parking on disney property saving $7 a time.