TIN interpolation out of extent

I have a problem with the TIN. I just started using QGIS because I no longer have access to ArcGIS. I want to create a reservoir bathymetry map from depth survey points. I created a polyline and converted it to a polygon so I can contain the interpolation within the shoreline boundary but when I run the TIN, it does not recognize the shoreline polygon as the extent and it interpolates out of that. What is wrong? Also is the TIN the most suitable for bathymetric interpolation or is there any other method? It is for Lake Diefenbaker 180 km long, more similar to a river than a lake. Thanks