I've found that you can communicate a lot with your builds. For instance, choosing not to build a fleet in Brest or London signifies to the power across the channel your friendly intentions. Generally, if it's an action that you wouldn't otherwise take, but it's something you think they might ask for if they could talk to you, then that's a good way of communicating.

Voting Cancel when you've misordered, or made a foolish stab that you'd like to take back, has worked for me in the past. But it's not guaranteed that they'll see it.

Incidentally, I've often wondered -- is the use of the vote buttons during a gunboat considered metagaming if it's used as a sort of coded communication?

I've never seen that happen

What would voting draw signify beyond wanting to draw?

I don't think it does signify anything beyond wanting to draw.

Good players can use it too develop coalitions

I played a recent gunboat game where i was close to a solo win - everyone ganged up on me to prevent it and at some point they all voted for a draw. they not only prevented the solo but pushed me right back to around 6 centres and i was potentially facing elimination. So i voted for the draw to ensure i survived.

I'm not sure if the voting or the draw was the signal to gang up on me - as good players should be able to judge it themselves. but you need to be careful about voting having unintended consequences.