I had a Bav44 2002 until recently and also found a small crack in the fwd water tank. In my case it was at the very top where there is a re-enforcing 'trough'. it was at one of the top corners of this trough and only came under a small amount of pressure when filling the tank to full and/or when the tank is full and heeling when under sail.
I could not find a glue solution I could be confident of and even welding would have been impossible without removing the tank, not impossible but undesirable, so I fabricated an angled plate from aluminium to fit over the hole/crack with a surrounding cover of about 20-25mm in each direction then used a sealant
like plasticine which I could place between the plate and the tank and then screwed (threaded screws with nuts and washers) the plate down onto the tank. Luckily I could reach inside through the inspection
cover to put the nuts and washers on.
I am confident it will last but it was only a pin sized hole not a split and the plate should re-enforce the area.
Maybe your situation could be resolved in a similar way.