There may be many reasons why your dog is afraid of water. Early experiences are important for how a dog learns to respond to different aspects of his environment.
Lack of, or an unpleasant, experience can result in extreme fear. Dogs who are fearful of water should never be forced into it. The sea can cause fear due to the expanse, the noise, and the constant movement.
While out, you can offer praise and rewards when your dog shows any interest or even steps towards water. Try to get his attention before he becomes fearful. This could be with his favourite squeaky toy so that he is having too much fun to remember to be frightened. Over time he should start to feel more comfortable in those areas but be prepared to practise and take your time.
At home you can teach your dog to relax in his bath by taking lots of time to firstly make sure that he is comfortable in there without water. Place him there, and offer fantastic treats. Repeat this regularly so that he begins to relax. He may be more comfortable building up with small amounts of warm water poured from a cup. If you intend to use a shower head to wash him then you should get him used to this moving and coming near to him without the water too. Make sure that water is warm and avoid his ears and face.
Continue to offer him rewards for tolerating the water and stay calm and friendly throughout. Keep sessions very short and build up as his tolerance increases. Thankfully, most adult dogs don't require many full baths a year, so you can work up gradually.