On the other hand -
I think 12 hours is optimistic for your boat. Sea buoy to sea buoy maybe, but you will need a few extra hours to up/down anchor
, go in/out the inlets, etc. If you're new to cruising trying to navigate into an unfamiliar anchorage after dark, pick a spot to anchor, and make sure the anchor is down securely can be stressful at best. Hell, it can be stressful even after you've been doing it for years. It's all much easier in daylight, especially if something goes wrong and you have to redo it. In Jax you might consider anchoring
north of the Sister's Creek ICW
bridge, on the east side of the channel. This gives you a some room to get a bit out of the current
, although you still might consider two anchors set up and down current. Leave yourself enough swinging room in case the wind blows you toward shore.
The real drawback to this plan is that it cuts into your time at Cumberland Island, which really deserves two days itself.
As other posters mentioned, the only thing to worry about with tides is to avoid strong winds opposing tidal currents. Normally that's not a worry. When passing through long inlets like Jacksonville or St. Marys having the current with you can make a lot of difference timewise.
Familiarize yourself with the charts
for the St. Marys inlet/Kings Bay sub base area. Be prepared to find some differences in the markers. Be very cautious about exploring north of the sub base, I believe there has been shoaling reported.
Otherwise, take your time, relax, and enjoy!