A shoal definitely is an obstruction to a boat that draws more than the shoal depth. Therefore, RRS 18 might apply, but you don't give enough information to tell. It sounds as if the two boats in question are on opposite tacks on a beat to windward, though. If so, then rule 18 does not apply. The port boat would be obligated to keep clear and apparently did not.

If the starboard boat immediately hailed "protest", flew a protest flag at the first opportunity, and filed a written protest that accurately identified the incident within the protest time limit, then his protest would be valid.

Regards,
Eric