i think with a waverunner you should be fine. most of the stories i have heard were people towing too much weight.it will be easier to launch if its automatic. if you are really really really worried about it put it in 4lo when going up and down the ramp.
2003 Rubicon, 4.0L, 5SPD, Custom 4.75" lift, 33" MT Baja Claws
1999 SE, 2.5L, 5SPD, sold it
PM me for wheeling in and around Northern Illinois