That is a long ways for the transom to drop. The bow would be pointed to the sky!
More likely this was due to the following seas that are common in the entrance of a pair of jettys.
If your engine was running, you're ok. Although the engine doesn't like to have the exhaust submerged (it raises the back pressure), a brief amount like you describe is harmless.
It's of somewhat more concern when the engine is not running. A poor, or marginally designed exhaust system could allow the following seas to ram water up the exhaust and back-flood the manifold.