Fully submerged timber won't rot in the traditional sense, as the fungi and mould itself needs oxygen as well as the moisture. Not to mention the right temperature.
You can see in cut sections of deckboard and posts that where a green or brown tinged treatment has been used, the core of the timber remains it's natural colour.
Anyway, go with the Ronseal if it's more convenient/cheaper. They will probably be good for 10 years even without treating, so don't lose sleep over it. (As I have by typing out these three paragraphs!)
You can see in cut sections of deckboard and posts that where a green or brown tinged treatment has been used, the core of the timber remains it's natural colour.
Anyway, go with the Ronseal if it's more convenient/cheaper. They will probably be good for 10 years even without treating, so don't lose sleep over it. (As I have by typing out these three paragraphs!)