This is important for your enjoyment when you go out on adventures. Go for an extended swim in a pool. Only then can you tell if it fits well and feels good. Practice swimming in your overall to keep fit.
If you need a reasonably waterproof suit, make sure it holds up to prolonged heavy showers or spray. Wear a thin cotton hoodie and pants underneath so you can see afterwards where it leaked.
The best way to test this is under the shower at home or with the hose pipe in the garden. This is great fun and you may want to try this with all your waterproof clothing.
You will find that if you face the direction of the rain or spray, even with the hood up, some water will enter the suit around your face. If you don't use a hood, even more water will soak in around the collar as the catchment area is larger.
You may find that no suit is completely waterproof, even if the fabric and seams hold up,
some water will seep in through the head, or hand and foot openings.
Add to that condensation, and you realise that you will get a bit wet or damp in any case.