HMS Mount Stewart and the Royal Marines Boom Patrol Detachment
This secret unit specialising in irregular warfare consisted of servicemen from the Navy, Royal Marines and Army under the command of Captain J Brunton RN, leading 35 officers, 60 ratings and 25 WRNS.
Their headquarters were at the Royal Hotel on the Den and their teams, including underwater swimmers, worked in buildings on the Eastern Quay and garages on Lifeboat Lane. The equipment being developed and tested was as follows:
Mobile flotation units (MFUs)
29 foot and 59 foot long, part submarine and part surface craft with diesel engines. The crew of three submerged the MFU, then, under cover of darkness used 2 MSCs to set limpet mines on German ships. An infra-red lamp guided the crew back to the MFU which would surface automatically at a pre-arranged time.
Motorised submersible canoes (MSCs)
A 9 foot long craft, which ran on batteries and carried limpet charges. The driver wore a diving suit as the craft moved underwater. 
The MSC was designed by Major Hugh Reeves, R.E., who was also given the task of designing an 'unspecified device' for an underwater approach based on an idea from Lt Col "Blondie" Hasler which he called the 'underwater glider' and developed at Aston House (SOE – Station XII E.S.6.(WD)) to Hasler's specifications.
The craft got its nickname "Sleeping Beauty" when Reeves was found sleeping in it by a passing officer. 
Explosive motorboats (EMBs),
A 12 foot long craft. These were dropped from Lancaster bombers with a driver inside and 3 parachutes attached. The helm was locked on towards its target and the driver would climb onto teh stern which would cut away like a surf board where he would make his escape.
On VE Day 1945 in Teignmouth, Devon, the officers and men of the Special Service Establishment HMS Mount Stewart participated in a Victory parade through the town.
HMS Mount Stewart was closed after VE Day. Some local people remember some small unusual craft lying in the Council store at Gales Hill for years after the war. 
Hear first hand accounts by search RMBPD in the Audio and Video files here
 Teignmouth and Shaldon remembers WW2 - HMS Mount Stewart, Teignmouth shore establishment 1943-45
 Wikipedia - Motorised Submersible Canoe