Antennea will not cause a volume issue, this is a real Red Herring.
I have seen this many times and is more likely to be that the unit has got salt water on it, has not been hosed off, salt water has dried and that salt crystals are stopping the speaker from moving correctly.
It is very important to clean you radio after a day at sea or this will happen.
I have SH kit on the boat and alway run fresh water over the radio's to clean out any salt water. Have done this with Icom kit to.
An waterproof radio worth its money should be able to cope with this.
DO NOT JET IT! just let the water run over the radio.
You may need to use a small tool now to push very gently on the speaker to break up the crystals, if at all worried about doing this send it away to be done.
It is worth, as suggested, trying and extenal speaker this will also confirm that the set is working ok.
Hope that helps