If you think about to buy the Samsung Galaxy S6 and do not know whether the smartphone is watertight, then this article should enlighten you about it.
The Samsung Galaxy S6 housing is not like the Samsung Galaxy S5 IP-67 waterproof. This means that when important electronic components are touched by water this can lead very quickly to a technical defect. Unfortunately you have to watch with the Samsung Galaxy S6, that it does not come into contact with water. Thus, the use of the smartphone in the bathtub should be avoided.
If it is very important for you that the new smartphone is waterproof, then you should in any case, have a look at the Samsung Galaxy S5, which also has very good technical performance but is unlike the Galaxy S6 also waterproof.