This is an outline of an incident in the Lord Beresford's Travellers campaign, written up in the hope that the situation will inspire GMs whose players enjoy a tactical challenge. The campaign is currently in 1874, and the PCs have the best modern weapon technology around. Their opponents are Fishmen, still using muzzle-loading black-powder guns and, mainly for underwater use, crossbows with barbed heads on the quarrels. Clearly, in an open battle above water the PCs have a vast advantage. You can see a writeup of my party's solutions at Into the Depths.