The mgb has quite a few drainage points that drains water that collects from the inside (especially the open top version) and from the sills and floor box sections.
These can easily get blocked with underseal and other debris.
a collection of classic car tech tips, tricks, ideas and information all about performance, modifications, maintaining the MG MGB and GT from across the Internet. Also tuning, servicing, repairs and mods on my own classic car a 1975 MGB Roadster