To answer that question, have you ever tried to swim with a 1kg weight belt? Such devices are commonly used by scuba divers to maintain neutral buoyancy.
Or put it another way, the difference in density between seawater and fresh water is about 3%, and it's noticeably easier to swim in the sea. So if you have bone replacements that make you 3% heavier, you should expect to notice it.
At least part of the ballast normally carried by divers is to compensate for the buoyancy of either a wet- or dry-suit, and for the buoyancy of SCUBA tanks (some are buoyant when empty, at least in seawater [1].) Treading water while carrying an extra kilogram would be quite a struggle for many people.
On the other hand, the article compares bone to metals on an equal-weight basis.
> the difference in density between seawater and fresh water is about 3%, and it's noticeably easier to swim in the sea
This really messed with my head when I swam in Lake Constance last year. I was so used to swimming in salt water than the different buoyancy in fresh water completely threw me off for a couple of minutes.
Yup, I have some muscular friends who sink like stones. I hear this is also the reason other apes can’t generally swim—they’re too dense. On the other hand, I’m very thin and have relatively low body fat, so I’m not very buoyant at all either. Filling my lungs with air is enough to make me float, though. :P
