We get asked quite regularly if there is a trim available to seal the panels to the top of a bath or shower tray.
There are some trims available on the market for use with baths/showers and wall panelling but they are quite bulky and rely totally on silicone sealant to form the seal - you will get a much neater finish by just using a good quality silicone sealant instead (we recommend Dow Corning).
Seal the bath or shower tray to the existing wall and smear the silicone up the wall slightly to form a "skirt" of silicone. Fit the panels over this and then run a neat bead of silicone where the panels meet the bath/shower. Should this front seal ever fail the seal behind the panels will prevent any leaks.
The bigget cause of seals failing and causing leaks or water damage is movement - either through poor installation or expansion and contraction. You must always ensure that fixtures such as baths and shower trays or rock solid as any movement in these items can cause silicone seals to fail (or cause grout to crumble if you are using tiles)