Which type of swimming who takes low power of man and gives the more mileage in the sea?

Your question is not complete. First you need to see how well is the swimmer prepared. For example if it is someone who is swimming only couple times a year in indoor pool then for sure is backstroke(hands doesn't need to go full stroke, and legs can work freekick or breastkick), or ofcourse complete breastroke. On the other hand if you have more experienced swimmer then for for sure it is freestyle and/or breaststroke.

I suppose you need to understand the most efficient swiming stroke which is Breast stroke

for me the breaststroke is the best

Swimming on your back because it is the most relaxing and one of the fastest