The biggest limitation stopping a lot of people from mastering the art of swimming in a lake, river, or ocean is fear of open water. It's totally understandable, given that large bodies of water are less predictable and, in general, pretty intimidating for many pool swimmers.
The good news is you are not alone. Many people struggle with thalassophobia (fear of open waters), and it's less uncommon than you think. The best of it: you can overcome it by following utilizing gradual exposure.
A gradual exposure approach should help you to cope with the stress of being in the ocean, lake, river, or any other open body of water. Face your fear a little bit at a time.
Start by swimming alongside the shore. Don't focus on the distance. Try to relax and embrace this new journey you're on. Eventually, the fear will fade, and you will be able to comfortably swim in the deep end.
Firstly, it would help if you did your homework in the pool. Step up your endurance game, master your sightseeing skills, and improve your overall swimming level. The more confidence you have in your swimming skills, the faster you adjust to open water.
Secondly, a gradual exposure approach should help you to cope with the stress of being in the ocean. Face your fear a little bit at a time. Start with shorter swims closer to the shore. Eventually, the fear will fade, and you will feel comfortable, like a fish in the water.