You are on dock water? If I had a guess? Someone touched the valve and partially closed it, or it broke being re-opened, or the hose got kinked before getting to the boat. This might be as easy as turning a knob or moving the lever!
If cold water is a problem, hot water will be a problem, as it comes from heated cold water. So I'd start with the cold water problem and, specifically, start at the dock spigot and work my way forward.
Is the dock spigot fully open? Try playing with the valve. Someone could have messed with it. Someone may have seen it dripping and turned it off, for example.
Does water come from the dock spigot with usual high pressure? If not, ask the marina about it. The valve could have gone bad, etc.
You mentioned an external filter before it gets to the boat? Does water from freely before the inlet? If not, I'd bet the hose is kinked, folded, delaminating, or clogged.
(I'd say the same if it was a volume flow meter, etc).
What about the filter's outlet? If not, the filter media could be clogged, the inlet or outlet or associated fittings could be clogged, or it could be otherwise broken. Or, if it has a service valve, it could be closed or partially closed or broken.
Does water come out of the hose on the boat side, before the water inlet, with high pressure? If not, I'd bet the hose is kinked, folded, laminating, or clogged.
Does the inlet of the water inlet/pressure-reducer on the boat side /look/ clean? Or is there debris clogging a screen? Or plugging up the ports going in?
If you can access relatively (for a boat) easily, is water coming out of the inlet/pressure reduce fitting at its output? In other words, does it spray and make a mess when the hose is disconnected from the back?
Follow that hose forward to the first device. It may be a valve, water filter, T-connector, manifold, or improvised manifold (bunch of T connectors). Does water get to the input side? If not, backtrack and find the problem -- you likely missed a device. If you can't find another device, backtrack to the inlet and check again. Disconnect it there.
Does water get to the back side of the inlet with normal pressure? If not, recheck the inlet and input side. If it does, blow through the output side hose, which is now disconnected at both ends (inlet and 1st device past inlet). Is there a clog?
Back to that first device. If water gets to it with good pressure, does water get past it with good pressure? Does it get to the output side that eventually leads to at least of one of the misbehaving devices?
If it gets to the input side but not the output side with good pressure, check the device.
For example, if it is a valve, is the valve set to open? Does the lever or knob move freely? Can you run something through it when open but not closed? Can you see through it?
Just walk forward from start to finish and you'll find it.
