Seafoam Islands lists 19 Pokémon across 20 encounter methods (grass (hoenn sound), grass, grass (sinnoh sound), grass (hoenn sound), and more), between level 10 and 50. 2 of them are on no other route's wild tables in this guide: Dewgong, Articuno.
Seafoam Islands in other guides
The same area in other ROM hacks on this site. Each one has its own encounter table.