For foods that should not be consumed undercooked, no, floating is not a reliable mechanism for determining if the internal temperature of a food is within the safe zone. All it means is that a sufficient amount of water has been converted to steam and either trapped or expelled that the food has become more buoyant than oil.
Only really safe way to determine if the food is safe is to use a food thermometer. The good news is that cooking times in oil are very consistent. Once you know how long it takes to cook a piece of chicken of a given size, for example, if you maintain the oil temperature you can cook the rest of the chicken to be cooked. Always check the largest piece, and I recommend continuing to check the internal temperature of the largest piece in each batch though.
For food that can be eaten raw, usually they are cut into small enough pieces that you can use the external doneness to determine the internal doneness.