Do you know the actual requirements for a Rescue-Boat/MOB boat for cargo vessels? It appears tugboats & windfarm CTVs (SOLAS-certified) do not have them, where larger OSVs and smaller ships do. Is this a 500gt break-point?
As you identified, it is because smaller vessels, under 25m are maneuverable enough & have a low freeboard to effectuate self-rescue. It is not because of tonnage.
Depends on if its a cargo or passenger vessel. Are CTVs passenger vessels?
If a vessel under 500GT does in fact carry SOLAS certificates than a rescue boat is required unless the vessel has received an exemption from the Flag State (possible). SOLAS Ch. III includes a size limit for life boats, 85m and below can forgo life boats provided they have sufficient liferafts, but does not include any size cut off for rescue boats.
For cargo ships SOLAS simply says “Cargo ships shall carry”
For passenger ships below 500GT, one RB. Above 500GT, one RB on each side.