Protesting on the water

You would indeed need to hail "protest".

61 PROTEST REQUIREMENTS

61.1 Informing the Protestee

(a) A boat intending to protest shall inform the other boat at the

in the racing area that she is involved in or sees, she shall

hail ‘Protest’ and conspicuously display a red flag at the first reasonable

opportunity for each. She shall display the flag until she

(1) if the other boat is beyond hailing distance, the protesting

boat need not hail but she shall inform the other boat at

the first reasonable opportunity;

(2) if the hull length of the protesting boat is less than 6

metres, she need not display a red flag;

(3) if the incident results in damage or injury that is obvious

to the boats involved and one of them intends to protest,

the requirements of this rule do not apply to her, but she

shall attempt to inform the other boat within the time

limit of rule 61.3.

I think you could argue this a number of ways, there is no requirement to actually protest a boat that has infringed the rules as a good sailor will acknowledge the fault and take their penalty, in this case bear in mind the requirement to get clear and then take the penalty.

Hence the "first reasonable opportunity" does not mean that you have to yell protest as your first words, it is acceptable to ask a boat if they will be taking the penalty when they are first able to do so, if they refuse then this becomes your first reasonable opportunity to hail protest. But you must use the word protest.