You have a few choices... Improve reception/transmission of weaker signals, amplify the signals or a combination of both.
The extenders suggested add range to the signal by placing a repeater somewhere between the two points. All it's doing is passing along he signal...Works OK if you can place the unit between the two end points with good signal from each. If the signal is weak at the boat to begin with, chances are the extender will have the same issues connecting.
Next option is to add a better antenna system to transmit/receive signals. Same principles apply to wifi as any radio energy like our VHF's. There are USB network adaptops that have better antennas and improve performance. Alfa is one option.
For multiple devices, I've used both the RogueWave and most recently tried out the RV Pro.
These units create a 'boat network' that can connect or bridge over to the shore station. They use a combination of an onboard router and a fixed mount antenna system to allow strong signal network around the boat and transfer the traffic over to the shore stations even if they are too weak for a laptop or phone to 'see'.
I've been at anchor a mile away from the marina and still been hooked into their wifi. With these types of systems. They are pricy, but in this case you do get what you pay for.