Thanks frackers, electrondady Bruce and Mary,
It's good to have some ears out there.
Yesterday I bought a wiper motor complete with wipers, checked it worked, and visited some harbour spots.
I managed to move the wiper once when mounted, but it didn't seem to want to udge after that.
The hrabour has an interesting feature - a floating pier. This bobs up and down, and handily has rubber wheels keeping the pier in place in relation to the post into the seabed. This means I have a nice vertical motion if I want, and also a (more inconsistent) rotational motion. This is of course not fast, moving up and down just 14 times per minute (around 15 cm each time, so double that if power is produced in both directions). Although it's a very small distance, the power is essentially limitless - whatever I attach to it will give way before before it is hindered from moving the whole pier.
I still want to try with the wiper motor... maybe I'll need a big capacitor to store some of the energy before transfering it to a battery or device?
And I'd like to have a go with this pier and wheel, either using the wheel, or the pier post.
I don't know where to get a linear motor, but that's clearly a good idea.