The B is often difficult to recognize on the deck plan, unless you know what you are looking at. I am not sure if they have added it on all ship plans. Here are two examples. The first is on Seaside, we had cabin 14148. At the time we booked, there was no indication on the deck plans that there was a tub. And cabins with a tub are only for two because the bathroom takes up more space than one with a shower, so not space for the sofa bed or bunk sofa.
The second is on Divina, we had cabin 8114. You can see the B better here, laying on its back. You have to learn to distinguish it from the symbols for bunk sofa, two uppers, etc. For instance there are some cabins that connect, one with tub and the other the two dots of a bunk sofa. Look at 14152 & 14156. The deck plan key on the website does not tell you what the B means. The deck plans on cruisedeckplans.com do. EM