Still hoping the folks who have sea trained their canines will chime in with the secret.
I'm not sure if there is a secret, but it just takes training. Certain things dogs will or won't do, and you have to be sensitive to where they are comfortable in the first place, it can't be somewhere too close to loud noises or strong smells, too confined or too exposed, etc... each dog has their own things they get nervous around. The better it fits what they would have selected, the easier its going to be.
What I did with mine was, relocate them to the spot you want them to go when they alert you that they need to go (you have to know your dogs signs), or after they made a mistake somewhere else, just like house training them. Being aware of the alert before they go takes patience and a lot of attention the first few times. If they make a mistake somewhere, I relocated them, and whatever part of the mess I could, to the astro turf, so they could associate going to the bathroom, to that location. If they did manage to make it where I indicated, or close, then they got a treat. The more repetition, the better, so it might take a little longer if you aren't on board with them daily. I lived aboard at the time, and both our dogs are working mutts, eager to please and easy to train. Your mileage may vary!
Thats what I did, it worked for us, nothing special really. At one point, a few months later, I ditched the turf, but I think if you had the space, you could do a little box for them, sort of a sandbox or litter box.