The official CAA document defining what constitutes an SSDR is
here.
A quick perusal suggests your plan will fall foul of the maximum takeoff mass limit of 330kg for amphibious craft (the FIB 582 is shown as having a MTOM of 406kg).
Also remember that the SSDR exemptions are only concerned with airworthiness; they do not extend to crew licensing, registration or insurance - you will still need to register it with the CAA, arrange suitable insurance and meet the applicable pilot's licence and medical requirements.
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