(continuing my goal of using every book as a D&D sourcebook)
The Riddle of the Sands is is a pre-World War I British spy adventure, written by a real British spy who was later EXECUTED FOR TREASON. I think this adds a lot of authority to a book. Imagine if Naomi Novik, author of the Temeraire novels, died by FALLING OFF A DRAGON. That would give her a little something called AUTHENTICITY.
The characters in The Riddle of the Sands spend a lot of time poking around the German coast in boats, and there are a lot of details about Admiralty charts and tide tables. Some sailing exploits, and sights, are only possible at high tide. For instance, there’s an abandoned church on an island: at high tide, the island is underwater and the church can be seen sitting on the surface of the water.
Imagine the temple crypt below such a church. A macguffin entombed in the deepest part of the crypt might only be accessible at low tide, for, say, an hour; soon afterwards, water would start flowing in, giving the DM a great excuse for the classic rooms-filling-with-water hazard. On Earth, it takes about 6 hours for a tide to come in; so in a reasonably-sized dungeon, the PCs won’t be in danger of drowning unless they get caught in a trap or there is some sort of delaying terrain. This dungeon should probably have both.
What monsters would be in such a dungeon? In order for water to flow into the crypt, there must be an outlet to the sea somewhere. Mermen and other aquatic creatures can definitely be hanging out in whatever portion of the crypt is currently underwater.
The currently-above-water part of the crypt is more difficult. It must only be inhabited with amphibious creatures – which, luckily, includes undead, the most obvious dwellers in a church crypt. Imagine a skeleton which has spent half its time underwater for hundreds of years. It and its axe would be trailing weeds and slime; it might or might not be part coral (depending on how much you like The Tempest: does that sea change stuff actually happen?); and it would undoubtedly have something gross in its eye socket, skull or rib cage: a dead fish or scuttling crab.
The rest of the dungeon would be pretty unpleasant too: the church itself, up to a few feet up the walls, would be coated with weeds and slime, and paved with muck and stinking fish. Every level down would be successively more unpleasant.