Hi Damaseck,
One consideration for the choice of tilt angle is wind loads on the modules and supporting structure. I am not familiar with the floating PV literature, so don't know what was considered in choosing the value you mention.
SAM's financial models used the total installed cost, so it doesn't really matter how you assign costs to the various categories on the System Costs page. One option would be to assign those costs to the balance of system equipment category.
Best regards,
Paul.