Great news that A90 is soon to be on the move again (love to see a photo or two!)
The insurance situation is one that you should check carefully as it will depend on your policy, and you need to consider both your own risks and third party.
My understanding is that third party risks arising from towing the boat will normally be covered by your car insurance policy although you should check that. Risks to boat and trailer would normally be covered by your separate boat insurance.
One final thought – Breakdown. You also need to consider whether you want Breakdown cover and if so check documents. Most typical car recovery policies I have found will cover small trailers but will not undertake to recover a 3 tonne trailer. So if you want some sort of recovery you will need to look around. (I have never bothered beyond my standard car cover. The one time I had a breakdown – one of the wheels fell off! – I sorted it myself).