The "worrying" dip has been blamed by Scottish Renewables on planning delays and a lack of detail on market incentives for investment in offshore wind.
Projects with enough energy potential to meet the annual demand of every household in Scotland remain in planning "limbo", Scottish Renewables said.
A total of £28.9 million was spent on offshore wind in 2013 compared to £63.6 million in 2012, according to figures obtained from offshore wind developers in Scottish waters.
Total investment to date now stands at £193.4 million.
Lindsay Leask, senior policy manager for Scottish Renewables, said: "Uncertainty throughout the industry is growing as none of the major projects planned for Scottish waters have had their planning applications determined yet, and the details around accessing market incentives are still unclear.
"With around five gigawatts of potential offshore wind development in planning, enough to meet the annual demand of three million homes, we can't afford to see these projects, and the investment decisions related to them, left in limbo.
"Scotland has all the ingredients to develop a world-leading offshore wind industry.
"Our exceptional wind resource and internationally-renowned skill sets make these projects incredibly attractive and competitive propositions.
"This is evidenced by the level of investment that is still coming forward from committed developers in spite of the significant challenges facing the industry.
"Yet a reduction of some 55% in a year is cause for concern not only for the sector, but should also be a worrying sign for the Scottish and UK governments too.
"We'll be working hard over the coming weeks with governments north and south of the border to ensure the developers gain the certainty they need to unlock further investment."
There has been more positive news for those companies working in offshore wind at a smaller scale and throughout the supply chain, according to Ms Leask.
She added: "We are seeing some really exciting developments around innovation and across the supply chain in Scotland.
"2013 has been an interesting year with the next generation of turbines being installed off the Fife and Ayrshire coasts, a £15 million Scottish Innovative Foundation Technologies Fund launched and Statoil given the green light to develop floating turbines off the coast of Peterhead."