19Kona:
If you want to add on a rack for a wave runner, I hope you have a tandem axle trailer. A single axle will have a max load rating of 3750 lbs, and its typical 14" tires/rims are rated at 1400-1750 lbs each, for a total rated load of 2800-3500 lb.
My old Glencoe 18' 455/12JC with a heavily modified 4" channel trailer weighed in at 3100 lbs on a certified scale. Add gear, fuel, coolers and I was at max load limit, easily.
Adding a rack with a davit/winch will probably add around 300-500 lbs of weight to the trailer, plus the weight of the waverunner. So, you can easily add 1000lb to the total rig weight.
Also hope you have surge brakes.
I recently checked with Kustom Trailers in Norco/Corona about adding a rack to a Sea Ray 18' trailer for a friend. Dan (owner) does a first class job with power winches, etc. For a double rack with a ladder, winch, batteries, etc. I believe the cost was around $4K. So, unless you are handy with a MIG or stick welder, this won't come cheap.