Just my 2 cents, I would either book in advance with reputable companies (you can ask about specific ones here) or bite the financial bullet and do it through the ship. Some of those hustling people as you get off the ship are legit and some are not. It does depend a lot on which ports you are talking about. But, we have had several less than safe "adventures" with just winging it with a local vendor when we got there. When you are with the ship tour, the vendors know that anything they do will lose them business in the future, so it's not worth it. When you are on your own, that does not apply.
I am pretty adventurous by nature and DH even more so, but we have learned some hard lessons.