In order to improve the feasibility of attacks from the waterside (and to fulfill a certain bargain), you can now use scuba gear on your mercs. Scuba gear consists of three items:

If you wear the first two items and are in deep water (the swimming animation is shown), you will now see a small animation over your mercs head. This indicates that your merc is underwater and cannot be seen by the enemy. Thus the enemy wont react to your merc, until he gets to lower water, where he will be seen. However, if your merc is seen in low water, retreating to deeper water will NOT make the enemy forget you, so no exploit there. I strongly recommend using the scuba fins, as they reduce the AP/BP cost for swimming by half. However, on land they double the movement cost, so don’t forget to remove them on land.