Well Sanji Blue Walk is indeed useful but who is saying he would like lose it ?
We have been showing now that DF users aka Luffy and Robin were able to use there powers in the sea yes with great deal of strain on them but Sanji is one of the members who can take a beating ! So if he were to get the Mera Mera Fruit then I imagine he could still use Blue Walk in the water.
In those instances Luffy and Robin only had their limbs exposed to the water. A df user goes immobile when half their body is submerged. There is no way to use blue walk as a DF user, the user would drown since blue walk takes place under water.
Sanji doesn't need a df, he is fine as it is. It would also be redundant to give him more fire powers, and it would undermine the flames of his eroticism Lol.