Sanitary code regulations for freshwater to a toilet/WC include that the water supply must
have an 'air gap' to prevent retrograde contamination .... The supply water MUST fall by gravity through this 'air gap'.
1. Your current 'shower head' fulfills this requirement perfectly
.... the supply water falls by gravity through a LARGE 'air gap' to the the toilet.
2. For concept purposes - Your household toilet + water tank system also has this 'air gap' to prevent retrograde contamination of the inlet water supply.
3. For a sanitary code-similar installation, you'll need a separate water tank with integral 'air gap' ... and good practice is to supply something similar a toilet tank valve in a similar water holding tank similar in construction and design as your home toilet/WC.
FWIW - A "Sloan Valve" would fulfill this requirement; but, boat water systems dont have sufficient pressure nor enough volumetric flow to operate a 'Sloan Valve'. However, you might want to check with https://www.sloan.com/commercial-bat...s/flushometers
as there 'may' be teeny
Sloan valves currently available due to enviro low flush water / low head toilet requirements .... downside is you'll probably need a much
LARGER holding tank.
Obviously you NEVER EVER want to direct connect
your freshwater system to your on-board head/toilet without an air-gap ... OR you risk dysentery, typhoid, etc. etc.