Saltwater diving opportunities involve primarily boat diving. Inshore  protected areas such as bays and inlets offer the easiest dive sites based on time travel and current. Provincetown Harbor has wrecks  dating 100 years old up to present day trawlers. Depth of wrecks start in the 20 to 30 foot range and go as deep as 90 feet.  There is a distinct thermocline at 40 to 50 feet, so appropriate wetsuits or drysuits are a must. Offshore diving is more advanced diving. Current plays a major factor and diving the slack water is important. Offshore wrecks offer larger fish populations and excellent lobstering. A lobster  license is mandatory as is following other local laws.