If the total income for the recreation property exceeds your expenses for the property, there is a profit. In this case, the rental income should be reported on the T776 for real estate rentals. If there is no intent to make a profit, but it is more of a cost sharing, it might not be reportable.
Where you really might run into problems, is changing the ability to use a summer cottage as a potential primary residence to avoid any capital gains when sold. By renting it, you may have changed it into a revenue property. You should discuss this with an accountant to see where to go from here.
