Totally agree- Is breasting up or moving a boat trespass ?
There are three categories of trespass: trespass to land, trespass to goods and trespass to the person. Clearly the last of these has no relevance to this scenario. It's also worth noting that a criminal offence by trespass is only committed where damage is caused; it is therefore unlikely that the scenario below could amount to a criminal offence.
Trespass to land
Trespass to land occurs when someone enters someone else's land without lawful excuse (mainly permission, but also necessity etc). Land, however, only includes real property, which as well as the surface, includes the subsoil, airspace and things permanently affixed to those things (e.g. a house). As such, crossing a boat in the way described would not amount to trespass to land.
Trespass to goods
Forms of trespass to goods are various but include, for example, moving them or making contact with them; stepping onto another's boat would therefore seem to meet this requirement.
Trespass is actionable per se (according to dictum of Lord Blanesburgh in William Leitch v Leydon [1931]), although obviously where there is no damage, only nominal damages would normally be available. It is also worth noting that although not relevant for this scenario, accidental trespass is not actionable (National Coal Board v JE Evans [1951]).
The claimant must have had possession of the goods at the time of the trespass. This is, however, does not stop a trespass being committed as even if the boat owner was not there at the time of the trespass, they would still be in "either actual or constructive" possession, or would have "a legal right to the immediate possession" (Lord Esher in Johnson v Diprose [1893]).
In summary, there would probably be a de facto tort of trespass committed by this act, although damages available would only be nominal unless actual damage was caused. The lack of damage would also preclude a criminal act being committed.