The Fishmen in the Sphere

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.
For the climax of the adventure they were led into an unlit tunnel with a guard-post halfway along it. The guards, in the darkness, could clearly see the PCs advancing with lights and take all the time they cared to aim before shooting. All the PCs could do was snap-shoot back at the muzzle-flashes from the Fishmen's first shots.They didn't even get that when the fishmen switched to crossbows (because they were faster to reload, nothing to do with the undesirability of muzzle-flash).
Once past the guard-post, the PCs came to a huge spherical cavern. The tunnel opened out at about the equator of the sphere. The only lights came from a building down below them (at the 'South Pole'), which was surrounded by water. Access to it was via a wooden walkway against the wall which spiralled down to the water's edge.
The Fishmen waited in the darkness, their eyes already fully adjusted. They used the even deeper shadows under the walkway to hide until the PCs were in a suitable position, then they stepped out, aimed, fired, and retreated back under the walkway to reload. The light at the building being too feeble to illuminate anything on the shore, there was no way that the PCs were going to see them except by their muzzle-flashes.
The PCs' tactical options are interesting. Go into the darkness without lights, risking boobytraps and melee combat with opponents they knew were big, tough, and skilled in melee combat and which they (rightly) suspected had better low-light vision than humans? Or take lights, and illuminate themselves well for the gunmen out there in the shadows? How were they to find these figures in the dark and deal with them?
Although this time it was run in an 1874 adventure, the same principle can be used in any setting where the PCs don't have access to either night-vision gear or parachute flares. The particular details of the spherical cavern and walkway are also optional - any environment where the PCs are in the dark against opponents who know the ground will offer the same difficult choices.
It wasn't till after the event that I realised that one fishman should have made the Stamford Bridge attack: hide under the walkway and aim up through a knot-hole till a PC walks over it and blast him up between the legs.
You can see a writeup of my party's solutions at Into the Depths.
Category:Adventures