I joined a beach fishing club to enable me and the kids to fish from the beach, the rules are very simple, a boat has the right of way and we must reel in our lines to allow them to pass. This is not always possible if a craft takes off at speed right through your line, this is why there are speed limits
However not all fishermen reel in so just remember that they can cast over 100 yards so its best to avoid each other if possible as at the end of their high breaking strain lines there is a large lead weight and a nasty hook.
I would have thought that the longest casting fishermen are beachcasters who can more than likely be identified as they normally have there rods on a frame pointing up in the air (see pic).
Having said all of this, the OP said he was fishing from his boat spinning, so it sounds to me that the jet ski was far to close