Construct a sea-cave that requires two low tides to escape
The human-form tribe members can enter into the innermost chamber (which can be quite large) on the day before the full moon. Regardless of the swimming prowess of the hybrids, it should be able to construct a system of locks that utilize the tidal range (0-16m/50ft) either as a strict water barrier, or that lifts some heavy floating material.
This shouldn't require more than medieval technology, and you say that the tribes are sizable. There is also a certain romance to it.