I do a bit of open water swimming & triathlons. For a wetsuit, get a proper swimming wetsuit. They are tighter than sailing / surfing suits and tend to have thicker thighs to provide your legs with more buoyancy & stop sinky legs.
Rivers can be lovely places to swim when slow moving. Reservoirs can get very deep, very quickly and be cold year round. Their volume means that whilst the top meter or so can warm in summer, below that remains cold and can lead to cold shock.
If you do swim in a reservoir, stay well away from the damn / reservoir outlet. Being sooked into the water treatment works would be akin to being fed through a cheese grater – not an enjoyable experience.
Open water is lovely though & there are many open water groups popping up on facebook – have a look round and find one local to you, pop along and say hi.