This^^^^^
Over 2000mm on a ladder and you need 'special' insurance.
If you have a rubbish policy, then yes. This is the trouble with generic policies that try to imagine and consider too many risks. My ten million quid PLI is perfectly happy with me climbing ladders - VERY tall ones as it happens. I go up and down the things all the time, because in my industry (entertainment) we frequently need access where anything other than a ladder is impossible. We work on platforms that have no edge protection, and we routinely do activities that many policies shudder about - heat guns, soldering irons, electricity etc etc. The safety record in our area is pretty good and therefore PLI to cover these risks is not expensive.
It's all about real risk and perceived risk. If you build landing stages, before you do the insurance (and to be honest, product insurance may be no good- because it is a self-constructed item, and therefore there won;t be any test data available) you should consider the risk.
To the layman (me) what are the likely problems? It could sink, it could pull away from the land if tugged, but the real likelihood is rot. The broads authority in my area build these things all the time, often with quite unskilled labour. They have a limited lifespan anyway, so it's probably not much different from building a shed for someone. Practical worst case is it floats away. The expensive absolute worst case is somebody stands on it, falls through it and it sinks with them attached. Is this possible? They'd sue you. If it was considered your fault it will cost. Is this a risk you can stand, or not? Lottery chances?
I build numerous small devices to do useful jobs. Often these get left in customers venues. There is a small chance somebody could use one inappropriately. I don't see a need for product insurance.
As with all things insurance, it's a personal insurance based on your own personal feelings.
I suspect that as a non-professional carpenter, if somebody asked me to build a landing stage, (and I wanted to do it) then it's really down to everyday c construction skills, scaled up a bit, with some research and a decent hire equipment budget.