This is my "submarine" project.  I though about this for a few years before starting.  I finally settled on a design that does not have self propulsion and does not have a pressure hull.  This is actually a dive sled, only you don't get wet. My boat will pull this along at about 10 kts at 100 ft.  A compressor on the boat will supply compressed air which will keep the sub inside pressure equalized with the outside water pressure.  Excess air will blow out and two people will sit inside and breath the fresh air passing through. You have to follow dive bottom time rules to use this.  Since it is not a pressure hull, the Coast Guard does not need to approve it.  There are two balast tanks, one at either end and it will take about 500 lbs of lead to displace the hull bouyancy but still be a little positive.  Sculling "wings" pull it down with forward motion. Stop and it comes up.  The body of this is a 30 inch diameter by 72 inch long piece of wound fiberglass pipe I bought for $138.  The canopy was custom made by Aircraft Windshield Co., Los Alamitos, CA for $500.  The design was done in Solidworks.  Besides doing fun things out at sea, this may have an application by Homeland Security cruising by ships to inspect their hulls before entering port.  Current plans are to use ROV's.  That would take forever.  You want one don't you.