Don't get cheap bolt extractors! I have found that dispite my somewhat OK mechanical skills, I have found that bolt extracting is a somewhat difficult task to do well. In other words to drill a hole on the old bolt shaft without taking the threads can be difficult. I am lucky, I get the (aircraft) mechanics at work to do bolt extractions. If you do a bolt extraction, be very careful doing the punch for the drill to start in and use a new, sharp drill bit so the drill hole is a good quality straight hole. Put bolt anti seize oil in the hole to free up the old bolt. Probably stand the hull up so the oil can penetrate.
Cheap extractors will break, compounding the problem and the frustration.
Basically the idea is to use the largest extractor you can withut having to drill such a large hole that you ruin the bolt hole threads.

Sounds like you are lucky that the bolts broke rather than some other part, like the hull!. Probably was designed that way.