Dave, RCW (Revised Code of Washington) 46.71.025, (whole reg attached) paragraph 2 states:
(2) The repair facility may not charge the customer more than one hundred ten percent, exclusive of retail sales tax, of the total shown on the written price estimate. Neither of these limitations apply if, before providing additional parts or labor the repair facility obtains either the oral or written authorization of the customer, or the customer's designee, to exceed the written price estimate. The repair facility or its representative shall note on the estimate the date and time of obtaining an oral authorization, the additional parts and labor required, the estimated cost of the additional parts and labor, or where collision repair is involved, aftermarket body parts or nonoriginal equipment manufacturer body parts, if applicable, the name or identification number of the employee who obtains the authorization, and the name and telephone number of the person authorizing the additional costs.
The difficulty is that you have already paid, and left the yard. You should have confronted the management with the estimate, referring to the RCW, and their own estimate which stated that you would be contacted if the estimate charge was exceeded, but that's water under the bridge at this point. (see below)
The only "additional work" they accomplished was to poor attempt to repair the damage their inexperienced workers inflicted on your boat. Not only should the yard have PROPERLY repaired the damage they did, but they should NOT have charged you ANY of the costs of the damage repair.
Just further info. The boat in question only had VERY light soft growth on it when it was hauled out. Easily removed either by pressure washing, or a soft brush and a hose. Yes, I have firsthand knowledge of this.
At this point, I would request an appointment with the yard manager, take in your original estimate, the final repair bill, and a copy of the RCW. I would let them know that you would like a refund of anything over 110% of the estimate, per RCW 46.71.025. Inform them that if you do not receive the requested refund, you will be opening a case with the Washington State Office of the Attorney General. They have been VERY responsive to issues I've had in the past.
https://www.atg.wa.gov/file-complaint
Believe me, no business wants to defend their actions to the State Office of the Attorney General, and in this case, their actions are not defensible anyway.
On the damage to the hull due to their negligence in sanding through the bottom paint, the barrier coat, the gelcoat and into the glass substrate, that would require a qualified marine surveyor to access the damage, and a reputable yard to provide an estimate on the proper repair, that that is a separate (although related) issue.
Best of luck in whatever you decide.
Note: RCW 46.71.025 deal with motor vehicles, which also includes motorized watercraft.
Add-on: If anyone wants the REST of the story, PM Dave and ask him what the yards recommended fix was for the incorrectly sized (large) shaft zinc(s) they were attempting to put on . . .