Belize City to Ambergris is easy and quick - water taxis leave regularly and you can opt for light aircraft transfer if you're flush with cash. I found the water taxi (speedboat) trip/s to be quite enjoyable.
I was working in Belize, and had no problem with impromptu get-aways to the Cayes at short notice. It was easy to source nice accommodation and arrange diving on arrival at the island. There's a dive shop every 50m along the beach. Of course, I expect availability will be dependant on the season - so 'spring break' may present more need for pre-planning/booking.
If PeterBJ is around, then you should contact him - I met him when I visited Belize and he ran a great operation. I don't know if he still runs a diving business there or not.
None of the dive operations/boats I visited were crowded (again, time of year may have dictated that) and all were friendly. Local Cayes reefs were enjoyable. I was given an insta-buddy for the dives, but that's ok with me. I just don't like cattle boats - but I didn't see that there.
The Blue Hole trip was nice - but I was very concerned about the fact that very inexperienced divers were nonchalantly taken on a dark, essentially bottom-less, 42+ meter dive. I didn't like being in that group, simply because I spent more time observing/supervising my fellow divers than I did enjoying the dive... on that occasion the DM seemed very negligent and unaware.
Likewise, I encountered no problems commuting/driving around Belize. I literally drove the length and breadth of the country (was helping the govt. there with the National Hurricane Disaster Plan). Belize City to San Ignacio is actually a very pleasant drive with decent roads. Be aware that inclement weather can severely effect that - but what else would you expect in a country that has a 'hurricane season'.
You can leisurely drive from Belize City to
San Ignacio in a 1/2 day (115kms)... stopping in Belmopan for lunch, or at one of the nice eateries along the road elsewhere. My personal favourite stop-off was
Cheers. It's on the highway and only 3 miles from
Belize Zoo... which is quite possibly the best zoo I ever visited. Cheers do awesome Lime Juice punch... I was really addicted to those.
There are some great historical sites around San Ignacio, including Caracol, and the town has a lovely vibe. For accommodation, there is a bunch of lovely jungle/eco lodges, spas etc.
I managed to visit some more remote Mayan ruins when there, still being uncovered and studied by archaeologists. But wouldn't advise any drives that left the metalled roads and/or ventured into otherwise jungle/remote areas. Most of the drive from Belize to San Ignacio is through savannah terrain. Nearer the Guatemalan border the terrain changes to dense rainforest; not only are the remote roads dangerous, you're surrounded by primary jungle...and there is also a threat of banditry in isolated areas. I only visited those places because I was armed, accompanied by Belize military and driving one of the British Army's most robust (and subsequently battered) Landrovers... You really wouldn't want to expose a rental car to the sort of abuse my vehicle suffered driving in those areas!