Ocean riding can be quite frustrating on a slippery stock seat eh ?
I've experienced that and had to compensate with spending more time standing with knees bent that I preferred, just to keep from slipping right off. The good news is its great cardio, and a lot of fun, but after 40 minutes, not so much.
so,,, I hear ya...
for starters, honestly two nice jet trim seats with a little texture, not full non skid would be an enormous improvement. their website sux but their product is top notch.
2nd maybe some spare hydroturf in the sides of the footwell for wedges, ankle support.
If you really want custom like more than extra foam, for example seat bolsters, (I can't think of another name for it right now) then u need to locate a shop that has experience rebuilding foam, jet trim could do that but I believe its quite expensive for custom work, I don't believe it's that hard if you know what your doing and have the right materials so don't discount a DYI but to do it right, you need a seasoned fella and being in CA, its just a matter of digging up the right person. I have a guy like that in FL but shipping would be redonkulous.
installing a precut replacement youself that costs 120-200 isn't that difficult, although a helper and some patience make it easier.
blacktip around 135, a jettrim i think starts around 175 for most smaller skis and goes up from there.
if you stay local a custom design should run you about 200-225 installed, but with added large foam inserts or whatever, probably would run 275-325+ each installed. you might have to search for someone to hit that price point as well, probably won't get lucky the first person you ask.
I have tried to stay local before and had crazy people throw $450 figures at me for a dam basic pwc seat cover with two colors, and more than one gave me those kinds of numbers