Thanks! I appreciate the feedback.
Deep-Sea Elves were more-or-less my inspiration for the mission. Including a special exception for them would require putting faction-specific rules in the mission, which I wanted to avoid; it would dramatically unbalance the game in their favor and pretty much make it a faction-specific mission, rather than a universally playable one.
I decided the fact Deep-Sea Elves already have the Strider ability army-wide (and thus can move freely through Flooded zones) was sufficient to represent their "home ground advantage" mechanically and thematically without resorting to a faction-specific ruling/exception for Submerged Zones.