In general, you should Seaway whichever card in your deck you want at the moment. If you can make an engine, just about any component works as you rarely need more than two or three buys. Certainly I would just Seaway whatever off a Mission turn.
There are a couple of exceptions:
1. If you are planning to or have already massed a card for some slog: stuff like B-crat, Squire, Horse traders, etc. can make for very nice slog decks.
2. If you want to threaten to pile the estates or curses. It can be worth it to buy Seaway just to double the +buys in your deck with a nice Hamlet or Worker’s Village play. The other time this can work is when you have Tr, Prssn, Rc, or Kc in the works.
3. Three pile threats. Like above, but more generally when you start getting low you can spend a 5 to make the other guy green ASAP. Seaway on the last village can be a nice way to control game end.