Hi Jesse - Although I am an enameller (specializing in nautical
jewellery), I'm based in "old" England, ie: the UK so a bit far for
your boat to travel!
0.3mm is the shallowest I work to. Assuming you want a transparent
colour, to get the depth of colour you want, especially blue on a
gold base, 0.5mm should be fine. In the UK you can get special
"enamelling grade" lower kt gold. This has less copper than usual -
ie: less chance of dark shadows (caused by firescale) appearing
under the enamels during firing. If you want an opaque colour, the
depth can be as little as 0.2mm as basically only one coat will need
to be applied.
All polishing, other than a final rouge buff, should be done before
enamelling and the stone setting after (although diamonds should be
able to take the heat of the kiln, it is not recommended "baking"