The best method to remove steel boat nails - spikes is to tack weld a carriage bolt onto the exposed end of the fastener (I use a small inverter welder and 1/16" rod to tack weld a 1/4" X 3" carriage bolt - I like the flat head of the carriage bolt as it mates to the pulling hook on the slide hammer). This trick works well - but be extremely carefully not to catch the planks on fire and or tear up the boat (watch for plank - frame movement). If the fastener does not want to move - leave it alone. If and when it comes out - inspection is straight forward. I usually use hot dipped lag screws to replace removed fasteners. Make sure to check the frame for condition and serviceability (does the fastener harden up in the frame).