Redsy, For myself I just accept the fact the conductors will be underwater no mater what I do.
Short of pressurizing the line, water will get in.
Use the wet location conductors as required by code, be careful not to damage the insulation on the install and IMO you will have a long lasting job.
Water can form in a pipe without leaks from condensation which may take months but can fill the low spots of a raceway.
Construction & Maintenance Electrician