There is not one single answer to your question since the speed of boat depends of multiple factors.

Fast, sportboats can reach top speeds close to 100 knots and even over while so called daytrip boats usually moves in a speeds between 6 to 15 knots.
As for cruisers, they usually sail in a speeds between 10 to 25 knots.

A knot is around 1.8km.
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