https://www.practicalmachinist.com/cutt ... oken-taps/Don't do the tig welding! Also don't smash it because it will likely mess up the embedding.
Sorta confused as to why you decided to tap the nut if you extracted the broken bolt pieces.
Should just be able to extract the tap the same way you got rid of the broken bolt. Next time use sufficient oil to cut the tap.
Caution. IF you "unbed" this nut you will be in a world of hurt for your rudder since I don't know of a good way to rebed a nut in this plastic boat (one of the reasons I don't like plastic boats). If you do manage to unbed it, you might look into hollow wall anchors in stainless steel if there are such things, but I don't know how much load that they'd take and cutting that large a hole will be problematic in terms of a permanent leaking hull.