To me, if multiple sets of rules require a propguard, there must be something to it or else it wouldn't be required. Don't know about IMPBA either but this is what the R/C Unlimiteds rulebook says:
All boats starting their engines in the designated pit area shall be equipped
with a “prop guard” intended to prevent accidental contact with the prop
from all directions.
From the APBA rulebook:
33-01 No engine shall be started or turned over on the tables or in the working pits unless the prop is removed or properly protected from accidental contact. All boaters’ starting stands should be equipped with propeller guards to prevent accidental contact with the propeller when the engine is started.
And from the NAMBA rulebook:
No boats will be started on tables or in the working pits with prop attached, unless it
is properly shrouded and protected from accidental contact. No boats will be started
in the working pits and then carried to the launching area.