Rats dependent on tide pools for their food would quickly begin to adapt to that situation.
The rats would become aware of the changes in the water level resulting from the tidal patterns in the pools and the fluctuations in food availability resulting from those changes. The rats would begin to gather around the pools at those times of the day when the water level was receding, stranding fish in the lower levels of water and making them easier to catch.
The rats would possibly adapt by learning to eat different types of sea creatures over a period of time. If the tide pools washed in large numbers of shellfish or if the receding tide exposed many clams or snails, the rats might learn to use tools such as rocks in order to get at the meat inside those hard coverings. The structure or muscle structure of their front paws could evolve to make such tasks more efficient.