I came across a brief description of an interesting civilian wet-sub concept during a visit to the Historical Diving Society in UK (website). The craft was designed by Detroit Testing Laboratory in 1964 and had a two-man side-by-side configuration with a partially enclosed canopy. Information was limited but it said that it was designed to remain stationary at up to 300ft for up to 10 hours. That would be quite incredible, and together with the headlights (what's the point???) makes it somewhat questionable. But interesting all the same and something of a pre-cursor to the classic SDV Mk.9 designed a few years later. I don't think it was an influence on the Mk.9, but the likeness is clear.

The design was likely the work of engineer and company president James Douglas Dow (1900-90) who was better known for a quirky two-seat open-topped electric micro-car. The 'DTL' car had a top speed of 15mph and range of 30 miles, and operated with two 1/3hp electric motors supplied by 4 2v batteries. It is not inconceivable that the wet sub would have featured some of the same equipment.