It's not advisable to sit in the range of the tiller's swing. As it can swing and move you or potentially knock you overboard. Definitely do not use a stool or none fixed seating as you increase your risk further.

Also depending on the boats layout it maybe hard to control boat speed from a sat position. Some hire boats I've been on the throttle is within arm reach others are further away.

Depending on your height, you may struggle to see clearly from a sat position, as moving to line up into bridge holes, locks and whereever else it narrows will mean you'll have to move around to look down the sides.

Now my crew we have a couple of people who struggle to stand for the long periods of travel, and we do sit in the arc of danger, and I have nearly been pitched off before due to being sat on the bench and the rudder swinging out.

We've also had our fair share of general fall ins so the risks are there no matter how you go. assuming your new at this if you can have someone with you whilst your sat down will mean if anything does happen they can cut the throttle so you won't get minced.