Have been across quite a few times. Our main interest is to have a super seafood lunch in the one of the many fish restaurants around the harbour area - you can walk from the boat and stagger back, if you are there on a day trip! You don't need a cab if you are reasonably healthy, imho.
There is a castle, a nice church, some interesting shopping streets and an opportunity to soak up a bit of provincial France.
See
http://www.ferrybooker.com/ferry_port/dieppe.a sp
There is a Dieppe Tourism site, but the English section is undergoing maintenance when I checked a few minutes ago.
Its not a place to spend a week at, but a day trip or day trip plus overnight stay, fine. If you stay longer visit Rouen - you can get there by train easily.