The white part in the middle as well as the one furthest right are check valves, the white part furthest to the left appears to be a strainer. The purpose of the check valves in your pictured set up is to prevent a pump pulling water from the other branch. For example one side may be for a live well and the other a wash down pump, without the check valve the wash down would suck air through the live well line when the live well is not in use. This was done as a work around to avoid having several thru hull fittings below the waterline. The check valves pictured are not marine use approved and I would not advise such an installation. I would replace with brass units, if this is not feasible at the time I would create a loop for each branch rising above the waterline and locate your PVC check valves at the top of each loop. Three other precautions would be: secure the hoses so they cannot flop around in pounding seas and break a fitting, install a second hose clamp at each connection point, secure the thru-hull valve closed whenever the systems supplied are not in use.
