Tactically: avoid them (use stealth; stay away from known merfolk areas), use speed (fast sailing ships, maybe able to out-endure not-so-fast swimmers), strong hulls/blessings on ship, or draw the merfolk and their marine creatures to the surface and hit them with magics+missiles (if not able to bring them on deck). Nets? Poison for the water?
Not sure what else unless they have elementals, special sorceries (scrying?), sea creature allies etc.
But yeah, people fear the sea for a very good reason, especially since the Closing. Even with Dormal's ritual you're eventually assured to run into merfolk. I like how ?Strangers in Prax? likened being a sailor in Glorantha to being an astronaut in our modern world. People think you're nuts for risking death in such an alien, hostile realm. The odds are against sailors-- but they seem to like it that way.