@kkende
Not sure how it is in the USA but tugboat operators in the United Kingdom usually require a fair bit of sea time before they consider training you for tugboat skippering.
Try and get some deep sea hours in first either on oil tankers, LNG carrriers, bulk etc. before going for tugs. Being a tug skipper is an interesting career, some of the new high performance tugs (voith & aquamaster Z-drive) are works of art.