A 1/4" BSP female plastic solenoid valve. It needs to be 12vDC, and to have minimal exposed metal (preferably none as the marine environment will kill it quickly). The challenging part is it must be NORMALLY OPEN. It must also be cheap (<8USD - purchasing 200 PA). It is vital to be normally open as if power is removed, there is a risk of sinking the vessel. That type of valve (the black ones anyway)is known as a pilot operated. The solenoid opens a small orifice connected to line pressure. You need to be careful in selecting this type, they usually require a few psi to operate. Assuming you connect the discharge of the valve to your syphon, the inlet will be at atmospheric you willl have a differential pressure equivalent to the height of your syphon. I can see it working once the syphon is established but I'm not sure you will be able to get it to close again once the syphon is broken and you no longer have a differential. The small ones look to be direct acting, they might work. I would be surprised if the metal parts are not in touch with the process fluid. As a general rule the higher the pressure the smaller the orifice.

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