I would use a water based sealer, and let it dry for a couple of days, cover anything that will be exposed to water. The water base is safer than many other sealers. I left a link so you can read a little more on it and make sure you make a nice drainage system
I agree, particle wood will expand and contrast through temp changes and moisture and break really quickly . You can still seal it with multiple layers , but it wont be a forever long fix. You could use it as a "Base" and screw marine ply wood into it directly along with the sealants to help ward off longer.