I am not sure about the dredging industry as it seems quite niche. I suppose you are talking about the dredging at sea which supports the maritime or oil and gas industry.

In the oil and gas industry,, if you are a permanent hire drawing a fixed basic salary going offshore for a long period of time, say a couple of weeks, you must ask for the offshore allowance or hardship allowance. Usually goes by day rate, say one day will get maybe one or two hundred or more ? It also depends on the places you go to ? Say Angola or Iran ? Dangerous places will entitle you to risk allowance. Not sure if it is applicable for your case.