Unless the landtower is very very high you are talking a 30 m boat aerial. I thought 25 watt ones were only good for about 80km so not sure how you will get 150km to sea????? Boat to boat offshore 50km we have got.
My answer is sat phone ( in waterproof bag) at that distance from shore but may be wrong. Plenty on here know heaps more about radio than me so I may be well off the mark