Semantics in the jargon-sense are simply a way of formatting HTML so that search engines can parse the data with some sort of context. Semantics in a broader sense is how you code your site so that other developers can pull meaning from your coding style.
Floats are simply a tool in your tool belt and have no semantic context whatsoever. Additionally, floats are so widely used that it would be incongruous to say that they shouldn't be used. There are plenty of frameworks that rely on floats as part of their integral structure (Bootstrap, Gumby, Foundation, Skeleton, etc).
I can speak, however, to the difficulty of using floats. For someone just starting with CSS it can be hard to understand what exactly is going on with floats. In addition, you have clearing and in-flow and out-of-flow elements. It can certainly be confusing. However, floats are absolutely a cornerstone of the CSS specification and I couldn't imagine doing my job without them.
Hope this helps!