Hullo Alan....
Auto body filler unless applied in a substantial lump
has very little mechanical strength
If the hull is reopening up when semi immersed suggests hull rigidity is compromised
With any form of application of a reinforcement blanket, getting back to the parent hull material is critical
Depending on the hull size & shape, after cleaning/abrading all surfaces, you could consider tilting the hull to create a void at the lowest point....have pre-prepared, pre-shaped fibre glass bandages available & lay these in the void....then pour a little resin & hardener over & partially filling the void
The above is just one option......but what ever you choose, the cleaning & abrading of the surfaces to bare wood remains critical...remember too, the water
is coming from the outside to in....so you also need to attend to this [after to the internal reinforcement & sealing]
Again depending on the hull size, you may find the additional epoxy resin may alter the waterline of the vessel...if this happens, you may need to shift
ballast [battery pack] a little to the opposite side of the hull
Good luck
..... Derek