Paul,
In the context you have described I think you need to ask the question which types of design will be most marketable rather than which model boats are most popular. In other words, who are you designing for?
As you are selling designs this means that they will be to scratchbuilders who are something of a declining breed these days so you need to choose something that will appeal both to them and to anyone contemplating scratchbuilding to maximise the potential client base. This usually means ‘simple and straightforward’.
Size does indeed matter, if it won’t fit on the back seat of a typical family hatchback then you have lost a lot of potential customers. Model Slipway designed all their kits to this rule!
Choose your prototypes with care, popular doesn’t necessarily mean practical in terms of what you want to do. Lifeboats are indeed popular but mainly as kits and semi kits since a modern lifeboat is a tricky project for a scratchbuilder and those willing to attempt it have access to the original RNLI plans.
Choose a scale for which commercial fittings are readily available. Some people will build hulls and superstructures and then look around for fittings to speed up the project or because they prefer not to tackle them. On balance, larger scales of 1:48 and up tend to be more popular than smaller ones which older people in particular are inclined to find a bit fiddly.
Try and find a niche that is not well served at present but which is still attractive to modellers.
Are you intending to draw up plans for existing vessels, design your own or a combination of both? In the former case you will need to go the extra mile to achieve scale fidelity but of course you are aware of this.
One area you might consider exploiting is small harbour craft where you can base your designs on generic types. I am thinking of craft like general purpose workboats that might also do a bit of towing, small ferries, both roll on and roll off or passenger only (such as they have in the Western Isles); various categories of fishing vessels (always popular and not too complex), a modern MOD police launch such as those operating in Portsmouth Harbour (very smart) etc. etc. All of these could tick a lot of boxes.
Hope these ruminations are of use! They would seem to tie in pretty well with what the others have said.
Colin