As mentioned and from my own experience, charged batteries that do not have constant loads, will stay fairly well charged for a long time. In fact I have not had the need to charge my batteries on my 208 before starting the motor in the spring after the winter layup. The batteries are not on any maintainer during the winter.
That said, a maintainer , AC or solar may help in some cases for example where certain devices are connected that draw small currents. Stereo memory backup, electronic water sensing bilge pumps, some CO detectors, some alarms and the like.
In cases where higher current draws are happening on a somewhat regular basis, the solar option won't really help. The one more common situation is a wet bilge caused by leakage perhaps, rain water , etc. If the bilge is pumping regularly, a battery can run down quickly.
So in normal cases, and assuming no current loads or just very small loads, your batteries should be fine for a few weeks easily.
If you don't have access to shore power at your dock but have access to AC service while you are at the boat ( extension cord approach), I would suggest a small plug in charger. A portable power pack is also a good product since you can keep it on the boat charged and should you fall asleep while anchored and run down a battery, the power pack may be enough to get you started.