Encryption works on sequences of bits. It can encrypt anything that can be represented as bits. It cannot encrypt anything that has not been converted to bits. Fortunately, anything in a computer, by definition, is, at some level, a sequence of bits.
Conversion of floating-point integers into bits and back is a common thing to do, subject to some standards such as IEEE 754. Some programming languages already offer functions for doing such conversion in a deterministic, well-defined way that maximizes interoperability; e.g. in Java.
As for key management... the most important point to understand is that encryption is not some kind of magic security sauce that improves security simply by being applied more or less generously. Encryption does not create confidentiality; it concentrates it. Suppose you have some (possibly big) chunk of data D, that you want to keep confidential even against people who can peek at the data where it is stored. By encrypting D (if it is done properly) with a key K, you have reduced the confidentiality issue to that of K: if attackers cannot see K, then they won't be able to see D even if they see the encrypted version of D. But you still have to think about how to keep K out of reach of these attackers.
For instance, if your attackers can take a dump of your database, full of data which was encrypted with K, and K is also stored in the database, then encryption was totally useless. Encryption can make sense only insofar as the key can be kept out of reach of attackers through some other methods (of course, the key being small and not changing often, it makes that task easier). Otherwise, encrypting all the data becomes, in fact, some elaborate dance that will spend CPU cycles, increase development costs and appease auditors, without actually making things more secure.
Encryption is a tool, not a goal. You don't encrypt because you want to encrypt (well, unfortunately, encryption is often applied for that exact reason). You encrypt because, within a given security model, you have determined that there is a confidentiality issue for data that is stored or transferred, and encryption can help in reducing that risk by mathematically moving out large chunks of hardware and software out of the exposure window. The model and the system architecture will tell you who (in terms of both people and system components) is supposed to access the data, and who should not; in other words, where the encryption/decryption key will have to be known; this, in turn, implies what kind of key management you need (number of keys, lifecycle of keys...).