Regarding HR sensors and measurement, I'm pretty sure that wrist based measurement does not work and is not supported when swimming. You have to use a chest strap with a monitor specifically designed for the purpose.

initially I was very weary of wearing a chest strap monitor but after wearing one for one or two bike rides I didn't give it a second thought, and now I wear it on every bike ride. They're quite useful too I think.

I've guesstimated my max. pulse and can now use the HR monitor to help me stay in the HR zone I want to be in, for example fat burning is supposed to be most effective at approx. 80 % of the max. pulse. It also helps me combat my ingrown laziness by letting me know if my pulse is too low when exercising. Furthermore, measuring the heart rate, my watch, or my bike computer can better estimate the amount of calories I burn, and give me suggestions for a suitable period for restitution. The latter especially I think is genius because some trips require perhaps 40 hours of restitution while others require only 24 hours, and without my pulse data I would nor be aware of these factors, and how my periods of restitution are affected by how much time I spend in my max. heart rate zone.

Finally, if you track your heart rate at rest over a period of training, you will have an indication of your level of fitness attained.

All in all, I think a heart rate monitor helps me a great deal when planning and getting the most out of my exercising, while still bearing in mind that these devices are not 100% accurate and as such you should only regard them as tools to largely indicate the parameters measured, not regard them as absolute truths.