The Land that Time Forgot by Edgar Rice Burroughs.

In 1916 the US Montrose is fired on and sunken by a German U-boat. However the survivors manage to sneak aboard and capture the U-boat.

In between subsequent coups, the British and Germans agree to an uneasy truce until they can reach a neutral port. But they become lost and instead arrive on the mythical continent of Caprona where dinosaurs and cavemen still roam.

Amid this savage, primitive environment they attempt to survive and repair the submarine