To get back to the original question, the OP was concerned about ready access to what he calls eco-tours. Plus good diving.
Hey, I was thinking about staying in Belize City so we'd be closer to some of the eco-tours. Is there any good places to stay there and can I find any good diving from that location. Thanks
Now, if all you want is inland tours, to ruins and/or jungle, you'd be best off staying inland. San Ignacio and area is probably your best bet. A wide range of hotels/resorts and prices, including some of the cheapest in Belize, and several excellent tour companies. You are however a long way from any diving.
For diving with the opportunity for inland tours you really need to be on one of the cayes, or maybe in the southern area between Dangriga and Placencia. This includes Hopkins, which has some very upmarket resorts. From the northern cayes, especially Ambergris, there are daily boat trips to the mainland to visit ruins like Lamanai and Altun Ha, and to do things like cave tubing etc in the northern part of Belize. These are all whole day excursions with relatively limited (though adequate) time at the destination. You can also fly to visit these places, though that involves road transport from Belize City. It goes without saying that there is also superb diving available from these cayes, on the northern (Belize) section of the barrier reef, where you'll find fascinating spur-and-groove coral formations.
To visit areas further south you really want to be staying further south. I haven't visited Placencia so I can't speak for the areas accessible from there for land-based eco-tours, but you're pretty close to the mountains and I'm sure there are interesting places to visit.
I have visited Thatch Caye several times, and perversely it's a good jumping off point for many inland sites - as well as offering superb diving in the Southwater Caye Marine Reserve (which offers wall dives instead of the spur-and-groove found further north), and at the right time of the year/month visiting the whale sharks at Gladden Spit. They are one of the few operators licenced to take you there. They have their own docking facilities on the mainland, about a 30 minute boat ride away, and several of their own vehicles that they then take you themselves in to visit some real attractions. No transferring to another tour operator, with the attendant issues of timing and paying - they do it all themselves. At appropriate places they tie up with a local guide - I've visited the ruins at Cahal Pech with them and the guide they chose, and learned a great deal at this most interesting site. Having your own vehicle and not being tied to anyone else's timing you have great flexibility, such as for example stopping off after visiting the ruins at Xunantunich to browse around the local market, or stopping off at a farm shop to eat delicious home-made yoghurt. And as they run their own boats back to the island, no timetable to meet there either. Paradoxically, I've had better and longer, and certainly more relaxed, visits to the ruins right at the Guatemalan border with them than I have had with conventional tour companies based on the mainland.
In case this sounds like a commercial, I hasten to add that I am totally independent and have no financial ties with anyone I've mentioned. I just like a good product when I see it!