And 1hp is actually 745.7w. It makes very little difference with regards to the big picture of whether something is feasible. Whether you tell someone the ferry

And 1hp is actually 745.7w. It makes very little difference with regards to the big picture of whether something is feasible. Whether you tell someone the ferry is going to need 20,000 solar panels or 5000 it's still way more than reasonably be put onboard. The solar factor usually employed in the UK is about 0.1 to get from nameplate capacity to actual production (accounting for hours of darkness and suboptimal atmospheric conditions) so it would be reasonable to assume your 300w is actually more like 30w. At least some percentage of solar panels on our hypothetical solar power ferry are likely to be in shade or not pointing directly at the sun. Then you have various conversions and charging losses to get the power into a usable form.