I'd probably use the radio on the boat, assuming it had one, and cry "SOS!" smile Failing that, then the cell, but that has its own problems: Depending on how large the lake is, you may have to use one of those special satellite cell phones (because cell towers only reach so far from shore), but I'm not the expert on that.

If ONLY I had a Kryptonian at my disposal . . . *sigh*

I have heard there are troubles of more than one kind.
Some come from ahead and some come from behind.
But I've bought a big bat. I'm all ready you see.
Now my troubles are going to have troubles with me!
~Dr. Seuss