If someone asked me what the most difficult habitat to photograph birds I would say woodland. The light is often difficult and you have the problem of not seeing the wood for the trees.
However, on reflection perhaps the sea is a bigger challenge, certainly when working from solid ground. The problem is simple - you need the wildlife to approach you. There's no easy answer but studying tide tables putting in the time and visiting repeatedly gets some results. Here we found a flock of Velvet Scoters feeding and socialising just offshore. Getting this close happens less than annually. They will be around for another couple of months but likely that's it for this year.