My friends hired a boat for a week, out of Ely, and were bored stiff within a day or so!
However the area they went to hasn't really got a great deal to offer. That's not the case with the Norfolk broads. If you do plenty of advance planning, I'm confident that you'll really enjoy yourself. Don't just limit yourself to what's immediately by the river. Be prepared to moor up for most of a day and use public transport to explore the area.
In particular, if you'll be in the Wroxham area, hop on a train into Norwich or go the other way for a day at the seaside (Cromer or, possibly Sheringham). Route map here:
http://www.nationalex...ORK-MAP%28B%29_A4.pdf
Think of sitting on the boat all day as akin to sitting in a holiday hotel all day. You'd be bored stiff. If, on the other hand, you use the boat as a mobile base for exploration further afield, you'll have a really great time.
Chris