I've just discovered exactly the same thing. Very low voltage/current (not sure which is the correct term!). Can't feel it with palm of my hand, but can on the underside of my arm.

Yep wouldn't think it would be good. Have emailed ctek direct bit no response yet. Interesting if others are getting the same thing.

I had the same thing happen on my glass boat, I had the cowling off the motor and touched my arm on the engine itself and felt a slight tingle. The isolator was off and battery charging. Wondering if there is some voltage floating round the negative/ground.

When you have the chargers connected, is the isolation switch turned off?
 I dont use a ctek charger..
I do run trickle maintance chargers...have put a double, fused marine cig socket unit in the top of the battery box direct to the battery terminals. This by passes the isolation switch  which means the rest of the boat/ engine etc is isolated when in charge.
 Or can run stuff when switch off

Rozboons comment above...
I feel like this can only be bad for an alloy boat.

 is  far more significant than just alloy boats but corrosion on boat wiring terminals, and the engines etc

Yep mine are wired directly to the battery, with isolation switch off.

I suggest, (process of elimination) remove and isolate battery terminals.. the +ve 1st.. check if have issue... then -ve and check.
If still there, ground the hull directly to earth.
 That will remove any static electricity stored in the hull. It does sound as if  likely to have a static/ capacitance issue
Now check..
 If still there, check battery top is clean, no salt moisture between the terminals, and check there is no water in the bottom of the battery box.
 If still there, dismantle and clean (if possible the isolating switch.
 A a side note is your isolating switch a double.. ie isolates both +ve and ground?
With battery now totally isolated, all that leaves
make sure the

The other thing is ... are you charging these boats at the same property?

I am not sparky but based on problems I have experienced with electrical systems before.

Could it be an issue with the earthing on the properties power supply?

What happens if you plug the CTek in via an RCD when charging?

Appreciate the suggestions. The voltage leak in only when the ctek charger is connected though. No issue or leakage once it's unplugged or even just switched off at the wall.

:Hunter: wrote: Nope two totally different properties, one running through an rcd with no issues

That's that theory dead then .

Seem unlikely that two chargers would be the problem.

I'd now suspect: -

(1) Quick connector as suggested by Kevin; and/or
(2) Leakage through battery casing.  What is the battery sitting on?

Another question: does this happen during all phases of charging? 

For example - does the problem disappear or lessen during the trickle phase?

Quick connectors but with no lights. Out of interest I tried using the alligator clips that came with the charger onto the battery, and still the same problem.

A process of elimination might source the issue...