Some electric toilets (only electrics) are designed to be connected to onboard pressurized fresh water. Instead of a raw water pump, they have an electrically-activated solenoid valve, that opens and closes to allow pressure water to come into the toilet bowl for rinsing purposes. In addition to the electric valve, an atmospheric vacuum breaker is also supplied with these toilets (installed in a raised loop, higher than the top of the toilet). This is intended to protect your potable water from contamination from any sort of backflow from the toilet.
The SeaEra, and many other electric toilets can be ordered for use with pressurized fresh water - but they have to be originally ordered for that application - its neither easy, nor cheap, to convert a seawater-flushing toilet to one that will flush with pressurized fresh water. Not all SeaEra toilets can handle pressuized water; they must be built for that purpose. Note: all toilets that use outside water for bowl rinsing purposes are considered to be seawater toilets, regardless of whether the water is salt water, brackish water or fresh water from a river or lake. "Fresh water" toilets flush with pressurized fresh water only.
I don't know of ANY manual toilet currently on the market, that can be connected to pressure water, be it fresh water, brackish water or salt water.