Some of those providing places for visitors to stay said they had lost up to £1,500 because guests couldn’t reach them.
The problem follows a boom in ferry traffic fuelled by a cut in fares to provide greater parity with road travel.
It is thought record numbers are sailing on CalMac’s routes such as that between Ullapool and Stornoway on Lewis.
The number of cars carried on the route has soared by 88 per cent to 92,000 a year over the past decade.
Total passengers have increased by nearly half to 276,000 a year and are thought to have increased by up to another 5 per cent this summer.
John-Angus Morrison, who rents out three rooms at Taigh Solus in Arnol through Airbnb, said he had suffered significant losses because guests had been unable to reach Lewis.
He said: “This has had a very real financial impact on me since June. One booking for two nights from a visitor from Singapore was cancelled two days before they arrived because they could not get on the ferry. Others from Canada have had to leave early to get a vehicle space on the ferry.
“Some have been told there’s no guarantee of room on the ferry even for passengers. The ferry is booked up for the next three weeks and I’m starting to look at an empty calendar. It’s not looking good for the busiest time of the year.
“Last year, I always had someone in the rooms but I’ve lost about £1,500 this summer. I tell anyone making a reservation to immediately book on the ferry.”
Derek McPherson, who runs the Fernlea guesthouse in Stornoway, has also seen guests having to change their bookings because of full ferries. He said: “If there were more sailings, more people would come, which would definitely be a good thing.”
Margo Maclean, who operates the Twenty Seven bed and breakfast in Stornoway, said: “The ferries are full at the moment and tourism cannot grow.”
She said an increase in capacity would be a catalyst for extra accommodation to cope with more visitors.
Maclean said: “If there is a demand for it, people can go into tourism.”
Ian Fordham, chair of Outer Hebrides Tourism, said: “Eighty-five per cent of our visitors arrive by ferry, so it is clear tourism growth in the Outer Hebrides is constrained by the current ferry capacity.
“Though it is unclear how we will make progress given the complexity of the organisations involved, we continue to try to engage with the Scottish Government to find rigorous transport solutions for the Outer Hebrides’ future.”
A CalMac spokesman said: “It is great news for the communities we support that so many more people are choosing to sail with us. Tourism is the lifeblood for many of our isolated economies and we are delighted to be able to help facilitate its growth.”