If I fell into water in my SUP dry suit and I need to catch my board, or swim to the coast, which technique is the most effective? I've found it very hard to crawl because of the buoyancy of the suit. Is the frog kick the most effective, or there are better techniques?
This will depend a lot on the fit and type of dry suit, and how much air it holds. Ideally there should not be much air inside. Too much will make it difficult to do any stroke.
You should be able to perform a basic crawl, albeit with low arms, even with some air in the suit, but if there is a lot of air, you are better off rolling onto your back and using frog kick.
Pull the leash to get the board back to you.
This is one of the safety fundamentals for paddleboards, have a leash, use the leash. When you fall off the board you pull the leash to bring the board back to you and climb on.
Actually swimming is made even more awkward by having to hold on to the paddle while doing it. Use the leash.
You should be "burping" your drysuit to remove excess air. From NRS:
You want to evacuate, or “burp,” excess air from the suit. A puffy suit is cumbersome when paddling and can be dangerous if you were to get caught in a hole during a swim. One way to do this is to hold the neck gasket open with your fingers, then squat and scrunch your body, pushing air out. Release the gasket before standing up.
Another method is to get into the water and let the water pressure force out the air. This is really a good idea; it not only most effectively removes all the air, it’s also a safety check to ensure your suit is fully ready to protect you in an emergency swim.
This way there will be less air in the drysuit and it will be easier to swim. Leave a little bit of air for comfort and warmth though. Aside from the swimming benefit it is also important from a safety perspective in that having a lot of air in the drysuit can have it rush to the legs, flipping you upside down and drowning you.