There’s lots of threads here about potting compounds but I couldn’t find what I wanted.
I want to pot some PCBs (all SMT parts) that will be used submerged in water.

The PCB is around 10mm x 100mm.
It will be inside an open ended stainless steel tube. 
Potting compound needs to be reasonably ‘runny’ to get into nooks and crannies and expel all the air.
Will be done in batches of 10 at a time (total 150CC of compound).
Macromelt / Low Temp over-molding would be nice but is too expensive for this job.

I tried one unit using a rigid black “electronics” epoxy with a CTE=75. The unit worked fine until I rapidly thermally cycled it from 0°C to 90°C. 
I think it died due to surface mount parts being fractured from the PCB. 

I’m thinking of perhaps a two part silicone or polyurethane with a bit more flex (even though some seem to have a higher CTE than epoxy).
Maybe something with a Shore of around 40A-50A??