Probably it is better if it is the player's units that retreat to the water hexes, because you will have better control (moral control, protection, smoke cover, etc...) over their reactions.
Leave the Rebels(AI) to retreat towards the land hexes as you will have a better overall result.
cheers,
Pyros
p.s It is weird
... how did you manage to create "beach landing game conditions" without using the beach creation tool or using the SHIFT+$ function??
