First question: What is your budget?
Second question: What is your answer to the first?
Seriously, that is too broad a question to get anywhere near a good answer.
I went to boat shows for many years, looking at boats that i realy liked but could not aford. Once I figured out how much i could spend it narrowed my choices down tremendously.
Once you know how much you can afford, double that figure. Going from a 18' Hydra sport to an offshore fishing machine is a big jump in price,fuel, maintainence, etc.
After you have answered the first question, you can tell us if you are looking used or new.
Help us out by giving us more details.