Having rocks actually move is kind of hard, but here's something you can try:
- Add some rocks (for example, boulder falling) into water by moving them over from land or placing them on a mini-island in a discrete way
- Add some Krakens controlled by a dummy computer player that is enemy to everybody, but that everybody is allied to
- Move the Krakens so they're hidden by the rocks
- Add lots of waves and some mist or rain to make it look stormy (and maybe hide what can still be seen of the Krakens)
- Create a trigger that puts all the Krakens on Stand Ground stance. Also, grant the Kraken-controlling player the tech "Relic Tail of Cerberus", which makes myth units use their special attacks more often, about 5 times
Since Krakens can't sink water heros in one hit but they can do this to normal boats, it'll look like only the Argo can get past the rocks without sinking. The Argo might still get damaged a bit from Kraken attacks but it's OK.
An interesting thing to try would be Sea Turtles instead of Krakens, because of their special toss attack.