Wet suit in warm water

If you're planning on wearing it for a HIM swim, I would definitely do a few longer session to get used to being in it that long.
That being said, will you be allowed to use a suit in warm water? I'm sure it's different in Europe, but here in the States the USAT cutoff is 78 degrees Farenheit.

At the race the water will be about 65.
It's where I live and train that it is nice and warm.

First of all: DAMN YOU for living in the Caribeans. I can't imagine how nice it would be to say year round water temp 79-82 degrees.

1. I would be very careful and watch for the signs of overheating.
2. I would never do any long or short sessions in OW without someone with me. You never know what could happen (cramp, shark!, jellyfish, overheating)

If nothing else, tie a float around you in case something does happen. It makes for good resistance training as a bonus.

there are a couple of pools, but they tend to be even warmer than the sea...

I've been able to swim for about 15-20 minutes in an indoor pool with my full wetsuit on, but then get too hot. Maybe you can get at least part of a swim in while wearing it, just to get used to it.

You may also consider "intervals". Swim for 15 minutes, then unzip, hydrate, and cool off. Repeat until you get your swim time in. Not quite the same as non-stop but will get you accustomed to it with less concerns of overheating.

If you've been training, you can do the distance. You just need to acclimate to wearing a wetsuit and the extra flotation it provides.

At those temps, you'll probably get hot but should be safe.  I'd just swim at an easier pace for parts of the swim than you might otherwise.  That's what I did a couple years ago when testing out my wetsuit in a pool that is kept towards the upper end of your range.  I was hot, but it was not completely unbearable to do for a few testing sessions.  On the other hand, it is doubtful that I'll ever do it again now that I've swam with it.