When they are tentative I like to get them into deeper water by using their own drive to our advantage whenever possible. (obviously starting young and adding water to your dog is not an option which is the correct way to build) Anyway, I like to take mine to a place where a brook or stream has some water deep enough for the dog to swim across and then I keep crossing, first just shallow enough for the dog to walk across......I want to keep putting the water between us, plan on getting wet that is part of it. If you can find a spot that is long enough with water deep enough just keep crossing and making the dog have to choose to be away from you or cross the water.......bringing other dogs and keeping them on your side helps as well.
DO NOT call the dog, in fact keep quiet, constant chatter and encouragement before the dog crosses is counter productive. Be patient, be consistant, stay calm and if the dog comes to your side a quick pat on the shoulder (not the head) and a quiet "good job" is enough.....continue your walk. We didn't build a great pyramid there, we only crossed a brook, lets act like it's not a big deal.
If the dog wants to stay on the other side....let him. If he wants to only cross at places where it's shallow.....let him. Just remember to use natures influence. Step away from the brook as if you were leaving to add some motivation. Watch the dog closely and cross the brook when he's distracted. Do not have some one hold him when you cross or use a fake method or trick......you will never trick the fear out of him, he needs to work it out himself. Give him the opportunity.
Also you can add to the motivation if the dog is bumper motivated by tossing bumpers into progressively deeper water. Sometimes this helps but it is a one trick pony.....it tends to focus on the problem and thats it. Adding a walk with all the things to see and explore at the brook.....crossing becomes only a part of the whole experience.