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Simply put, with the drain open, water will enter till it reaches equilibrium with the outside water level.

In most boats, this is only a gallon or so.

In some older boats with heavy stern weight, it can be on the deck.

Some owners expect the boat to be "dry" and install a bilge pump to keep the boat dry.

You should test your boat and determine if you think you need the pump.

Personally, unless the water level is on the deck, there is no need.