Rogation tide are the days before Ascension day which this year will be May 14th.
On Rogation Sunday (the Sunday before Ascension Day) the priest and church would process around the parish blessing the fields and boats in advance of the harvest to come. It was also a day when everyone walked around the parish boundaries, a visible way of knowing where one parish began and ended. It might not seem important now, but in the days before maps and googlemaps, knowing the boundaries was important and this was something everyone in the community needed to know.
At the weekend I was at All Saints, Poplar Church and during the afternoon they were going to walk the 6 mile parish boundary with lunch before they started and tea afterwards. In the past they would have used a boat to identify the river boundary markers since it is a parish almost surrounded by water.