Here's something I used when I was younger to be comfortable for a long period of time floating in the water (I grew up on the Colorado River and spent many summers floating for miles on it.)
Get in the pool and swim out to the deep end, relax and take a deep breath. Do you float or sink? You should float unless you are naturally negative by a lot! Hold your breath for as long as is comfortable, you are not trying to break any breath hold records, just a comfortable amount of time, like 15-20 seconds, when you exhale you kick your legs to keep your head above water. You can scull youor hands to maintain position, but dont use them to keep you afloat, you just waste energy. Exhale quickly and completely, then take another deep breath, not quickly like you are trying to hyperventilate, just a nice, full lung expansion breath. Up you come again with no exertion. Relax again and if you do this right and stay relaxed, you only have to kick when you feel yourself sinking, as you exhale. You get into a pattern:deep breath, relax, exhale, kick, inhale , relax. I find I can float like this indefinitely and most times just get bored doing it.
If you can find a pool that has a deep end that is deeper than you are tall, that would be ideal, but it doesnt matter.
I can actually swim the entire length of my pool by just doing a couple frog kicks (no fins, just feet) without using my hands to keep my head out of the water. I just kick when I exhale.
In salt water you are more bouyant than fresh, so it is even easier. With a wetsuit, you dont even have to kick to float