When I first tried to insure my pickup with a water tank, about a decade back, it was a no go.
If the tank was bolted down they wouldn't insure it because it was a modification. If it wasn't bolted down it was an insecure load.
Eventually, I connected with Gladiator and Adrian Flux who found underwriters that didn't have an issue and life progressed.
In recent years, and with the increased number of WFP equipped window cleaners, the situation is much easier but there are still plenty of companies that will not insure a water tank in a vehicle.
And to clarify the point about modifications: After lengthy conversations with a number of companies, it seems that a MODIFICATION is anything that a particular insurance company doesn't like on an arbitrary basis.
Some are very balanced and reasonable and will, after disclosure insure just about anything at reasonable rates with no special conditions.
Other companies, I've argued with, consider a modification to be ANYTHING that the vehicle didn't have when it left the factory. Some have objected to window stickers, the wrong stereo, different wheels and tyres, uprated headlights etc. These companies are obviously best avoided but they are out there and there are many of them that I've dealt with.
A final thought is off road cover, only usually applicable if you have 4wd. Most companies will not cover you to go off road for the purpose of work or otherwise, which I do daily. In my case, it had to be a specific request from me to the insurer to get cover.
Insurance companies attitudes varied from a simple "no" to a threat of the entire, annual policy being void if I were to ever drive across a field. Quite how they were going to enforce that is still a mystery
The insurance companies are a law to themselves and can make up their own individual and pedantic sets of terms and conditions.
All are different. Some may object to everything and others to nothing.
Everything depends on the specific company.
Just tread carefully.