Small powerboats. The biggest market for recreational boats is the 3.5 to 5m trailer boats. What people don't realise is while the market appears to be saturated with generic boat manufacturers, almost every boat buyer wants some kind of customisation that is often trivially cheap and quick to do with the right equipment.

The key skillset required is the ability to weld (for aluminium boats) and a garage set up to hold up boats while they're being assembled or customized.