You might be able to implement rules taking currents into account, probably moving all ships a fixed amount in a specified direction between each card placement, so you would need to take it into account when planning you sailing. Might make for an interesting challenge near coasts, islands, shoals etc. , then of course you could always take tides into account as well. Having said that the calculations might get a bit hairy as if the sailing wasn't tricky enough already (at least for me).