If you're still in Seafoam Islands you should be able to just start climbing ladders until you get to the top floor, then take the exit to get back out to the surface, as long as you came from the eastern entrance.
If you got to Seafoam Islands then chances are you came through Fuchsia City, and likely ran into Team Rocket along the way and got the key item Gold Teeth. Bring those to the Warden in the same city, in the houses to the right of the ledge the Pokemon Center is on, to get access to Strong Push and thus access to the entirety of the Seafoam Islands.
I suppose you potentially could have gotten to Seafoam Islands from Cinnabar, seeing as Let's Go gives access to flight as early as Celadon City, and it isn't difficult to dodge the trainers in the sea routes to get through unimpeded, but I imagine that you could likely still just climb up any ladders you see and make for the exit at the top floor like with the other side. I might try entering from the west side and check it out myself later to see what that needs.
