Heart Rate Monitor

During some great days last winter I found myself so out of shape I couldn't even make it to the peak. I spend whole sessions on the shoulder trying to paddle against the current and not catching waves. I have since taken action to get into better shape, and one part of that is cardio fitness. I joined a gym (climbing gym with weights and bikes, etc.) and started riding the stationary bikes. Those bikes have heart rate monitors built in and I got curious about my HR during other exercises. I bought myself a Polar FT60 and began playing with it.
I'm going to skip all the functions and featurs discussion and hit at the *heart of the matter. When surfing my HR goes from low 100s when sitting to ~175 when paddling for a wave, or back out with duckdives. When paddling for position or through a rip I'm in the upper 150s. After I'm done paddling my heart rate gets back down into the low 120s in about a minute.
Biking to work in traffic with stoplights, etc. gives me a very similar workout.
Here's the thing, the HRM wants me to spend the largest amount of time in the 135-145range but I don't really know if that's right for me. Surfing seems to be sprint and rest, so shouldn't my workouts be sprint and rest? For now I'm just exercising however as often as I can motivate.
*About using the HRM in the ocean. The HRM isn't perfect and doesn't always pick up my heart rate even on dry land. I tried to use it in Maui while swiming in the ocean and that didn't ever work for me. I have been able to get it to work while surfing, although inconsistantly. I think what's happening is that the wetsuit keeps most of the water off my chest so when I'm not duck-diving the HRM works. If I get washed and my suit flushes the HRM stops working until the water runs out. Or, if the HRM shifts around and slides down then it's shot for the session. One more thing, the HRM is a pod that attaches to a strap and is worn on your sternum. This doesn't work for surfing (paddling problems) but the thing works for me when I put the pod on my back.
I've been doing cardio with some weights for a few months now, inconsistantly, but I've found anything I do to stay in shape for surfing helps, and I think I'll keep doing what I can to keep improving. Can't get barreled if I can't get the wave.