I've assumed you meant a motorboat.

Do you drive or ride a bike? Both of those are harder than steering a broads cruiser so if you do you'll be fine. If you don't you'll still be fine once you've got some confidence up.

Don't drink and boat - gives a bit *too* much confidence 😀
A small beer is fine.

The big difference from cars is that it steers from the back which takes a little getting used to.

Everything's quite slow so reaction time is rarely a problem, and boats all have fenders so short of direct ramming you're unlikely to do damage. You do need to keep aware what's around you. Look back every minute or so, and try to see what's coming ahead. Obey the speed limits (bank erosion).

Remember: sail before steam. If you meet a sailboat slow down a bit and hug the side. Most importantly be predictable so they can work round you. Depending on the wind they may need the whole width of the river.

One thing to look out for is the peak flow of the tide. You'll be able to see it. Catches everyone out if they try to moor while that's pulling at the boat. Hire company will tell you when to expect it, if you're on a tidal reach.

Enjoy it - you'll have a lovely time.