Yes, they exist but are "painted as land".
I ran a tank into one after a 2 sector drive last night but somehow was able to back out after firing my gun to rock the tank while I was in reverse.
Yes...I was 1/2 sector from the lake or any marked water.
Ths happened near the bishop HQ last night before the reset where there is a lake beside it with a river heading toward the rear of the map.
Apparently this river also exists headed toward the front but it is not shown on the clipboard as being a river.
So.....we have rivers that aren't mapped that you cannot ford with a tank yet these same rivers are not navigable by PT boats......which run aground in them.
If a river is deep enough to float a tank or LVT, then it should float a pt boat as well.........it should also show up on the map.