Windex/Glass Plus always worked on the interior gelcoat areas. Same for the seats. Only problem is if it's been on the vinyl for some time it will leave a brown stain. Sun is the way I've dealt with it. It should bleach itself out over time.
The best way to get spider crap off your seats is to prevent the little bastards form getting there to begin with. If you have a mooring cover, hang a HotShot No-Pest strip inside. If you have a bridge cover or canvas enclosure over a cockpit.. hang one inside.
MIND YOU... the HotShot strip comes down when you board and your first priority is to air out the boat. DO NOT leave the strip up when you are aboard... it'll do it's job while you're away and kill EVERYTHING that crawls or flys on your boat. Spiders will not be able to reinfest the boat over the few days you use it.
Available at ACO, Lumberjack, Miejers, and Home Depot...
Thank me later............ "spider free" for 8 yrs and I'm docked in spider heaven (St Johns Marsh)