I just thought I'd throw out some information for anyone who might be interested in using the G4 while surfing or swimming in the ocean. For me, surfing is really the number one place I'd prefer not to have a low. Even when surfing with friends, you are still mostly alone when you are in the water; well except for the sharks. The G4 has, so far, been kind of a mixed bag when it comes to surfing but here is what I've found. Hopefully it is helpful to someone else.
First, the receiver isn't waterproof. This is kind of a bummer since cheap waterproof electronics have been around forever.
Second, I've had success bringing the G4 receiver out into the water by sealing it up in an aloksak bag. The 3.37x6.37 bags made for cell phones work great and they've actually worked perfectly thus far.
Third, I tuck the receiver into my wetsuit on the outside of my thigh. This keeps it secure and out of the way.
Fourth, the transmitter is really not a bother even on my stomach. Your mileage may vary but I don't ever notice it.
Fifth, so far the actual operation of the G4 is a bit sporadic while I am in the water. I don't ever look at the receiver while I am in the ocean (it's buried in my wetsuit) but when I look at the readings afterward I find that there are large gaps in the data. I can at least discern that there are peaks and valleys but the general trend is downward so the G4 has been at least somewhat useful.
Sixth, I haven't had a low alarm in the water but judging how it makes me jump when it goes off in my pocket I am guessing I'd feel it even in the ocean.
Last, I probably won't wear the device all the time, just enough to help me predict what is likely to happen on future outings.