It's a dock. There's a hard platform with floats under it... and the most important point... it's anchored into place. YES... you generally have that floating out past your "Boat Dock", and swimmers play on it... but it's still a "Dock".
A swim float is a dollar store thing that you put on kids arms. (swim with it's help) And a raft is a thing you blow up. (in an emergency, it could be a bunch of logs tied together)