Their dives will be the first from a new RIB that has been funded by the global charity World Animal Protection to help the marine-conservationists reach otherwise inaccessible areas.
Previously the group had been restricted to diving from shore, although even without a boat they had recovered some 8.5 tonnes of litter over the past two years.
The dive, out of Penzance Harbour, is the first in a series of 12 to be carried out from the new RIB.
World Animal Protection says that 640,000 tonnes of ghost-gear – more than the weight of the Titanic – pollutes the oceans every year.
Both seals and whales have been reported recently as caught in lost or discarded fishing-gear in Cornwall, says the charity, adding that seabirds, porpoises and turtles such as the critically endangered leatherback frequently become entangled or caught, causing infections, constrictions and sometimes death.
Fathoms Free recycles the ghost-gear it recovers, whether in the form of artworks or as kayaks, bodyboards and other ocean-related products.
Led by founder Rob Thompson, the divers will have a support-boat alongside the RIB at St Michael's Mount, and say that an alternative dive-site has been set up if the weather is poor.
Divernet – The Biggest Online Resource for Scuba Divers