If both people hold out empty hands, they try to push each other out and stalemate. If one person holds a club, you knock out the other guy as long as he didn't choose a die, in which case you get knocked out instead. If both players choose a club, they both get knocked out of the boat. If players fail to move a boat forward, either because of rolling even numbers, pushing someone out of the boat instead of rolling, or stalemating, the boat drifts backwards towards the harbor. The current is weaker in the inner lane.
Boats drift backwards a number of spaces equal to the current of that space.
If they bump into a boat behind them, they stop drifting. Since you can only move one of your pirates per turn, that leaves up to four pirates sitting idle each turn. If they're idle, they're not rowing. And if they're not rowing, their boats drift back along the river. So having multiple ships on the river at once doesn't necessarily make life easier.