Swimming should form a large component of any training program.

Most top countries all have swim programs that they would work to. Personally when training for worlds i would do at least 4 hard swims a week plus 2 recovery swims in addition to other stuff.

You will find differing views on whats the best kind of swim training for hockey.

Generally a good swim session is around 3 - 4km (less if you are working on speed) and involves a mix of strokes, drills and distances.

When making up a program focus each session on a different aspect, speed, technique, endurance or strength (mostly upper body), but include a small element of each in every session.

If you don't mix it up its a recipe for injury as well as bordem. Each stroke works particular muscles, mixing the strokes means that your body remains in balance. Backstroke is particulalry important to strengthen the back muscles (something uwh players often neglect).

If you want to make it more hockey specific try mixing in breathholds, i.e minimum breaths over 50m, breathing every 10 strokes etc.

If you want some more specific programs Meaghan in Barcelona may be able to help you. I think she is a former swimmer.