I've been kicking around ideas for a creek/river/lake scenario where the PCs start out a small mouse village/town near a body of water.
You could go full Moby Dick with large mouth bass and pike as both of those are known to eat small mammals they come across.
Here's one of the settlement ideas that I had.
Greenhome
Wooden bridge joins the settlement.
Industry: Fishermice with nets and rafts. They primarily catch crawdads, creek chubs and other small fish. Occasionally they might be able to catch a much larger fish.
Greenhome is located next to a creek with a small wooden bridge going over it that the mice maintain. It is made up of four small half buried burrows and one large covered round house made from what looks to be an overturned whicker basket sealed with dirt and clay. Near the creek can be seen a dock, various structures, and equipment for fishing, which is the primary method the mice of Greenhome use to make their living. They fish the creek for crawdads, creek chubs, other small panfish, and minnows that call the creek home. Sometimes a much larger fish slips into the creek and ends up hooked at which point the whole town goes up in a flurry trying to either bring the monster in or cut it loose before it completely destroys the lines. It has a small clay pot tea candle heater in the center of the large round house that provides a meager amount of warmth to those inside. All of the smaller burrows are connected via a main tunnel that also connects to the round house. There are about 3-5 families of mice living there.