A boost pump for a watermaker requires high flow, low pressure. These are surprisingly hard to find.
The Jabsco Third day mentioned is one option
A Johnson CM30 is similar.
Both are pretty expensive and the Johnson is hard to find in the US.
When my Johnson CM30 died on my 20GPH engine
driven watermaker I tried a CM10 (lower flow rate) and there wasn't enough flow to keep the low pressure switch happy. After trying a bunch of other pumps I ended up going with a much cheaper Bosch Automotive 0392022010 which can be found anywhere.
It's flow rate is less than the CM30 at higher pressures but it works fine to keep the Pressure pump flooded even when my two pre filters are nearing the end of their life.
All the pumps I mentioned are not self priming but as Third Day mentioned keeping a self priming pump that works with the flow requirements of a watermaker probably necessitates a fan so it doesn't burn out. Find a way to install your pre pump below the waterline.
I have since started using the Bosch in my Hydronic heater
circuit as well. Both have been very reliable despite long continuous run times with hot and corrosive fluids.