Insurance for trespass seems almost entirely pointless.
Trespass, contrary to the signs, is not an offence you can be prosecuted for.
It is also not possible in the UK to insure against yourself committing a criminal act.
Trespass is a civil tort.
You can be sued for damages, but non-punitive damages. That means the landowner can sue for any costs incurred as a direct result of your trespass, but not compensation or punishment beyond a provable, direct cost.
This means they cannot claim for things like "I installed a new fence/security system to prevent it happening again at a cost of ?10,000" or "I had to conduct a survey of my entire land to check they had not walked across it at a cost of ?18,000"
Any interaction would go as follows:
Canoeist: "whoops, I appear to be on your land"
Landowner: "Please Leave"
<Canoeist decision: Leave or not leave>
If leave:
Canoeist: "Bye"
Landowner: "Bye"
<End of interaction>
If not leave:
Canoeist: "I'm not leaving"
Landowner: <Can use reasonable means to remove canoeist, including reasonable force>
At this point, it most likely becomes Aggravated trespass, and is then a criminal matter, therefore not insurable.
Note: I am not a lawyer, just someone who has had sharp-end experience of this in a "past life"
If I have mis-represented anything, or made error, I would welcome correction!