[Sailormann]

To say that something made primarily of "plastic" (the very definition of plastic is something that is not a permanently shaped solid);

it's not "plastic". It's polyester resin. Check out your chemistry. The colloquial concept of "plastic" as you are emplying it indicates a substance that is flexible. FWIW the usage of the term as a conversational adjective was subsequent to the introduction of the flexible chemical structures utilising crosslinked polymers and plasticisers to create a compound that is durable, elastic and economical. Plasticisers are organic liquids which dissolve in large qualities into solid polymers.

So while polyester resin which is formed from long chained monomers, may be included in the same chemical family as plastics based on its provenance, it is as fundamentally different from the flexible things we speak of as plastics as a frog is from a tree.

Polystyrene (PS) is rigid and non-toxic, with excellent dimensional stability and good chemical resistance to aqueous solutions but limited resistance to solvents. ... Products made of polystyrene are brittle at ambient temperature and may crack or break if dropped from benchtop height.

- NUNC

Glass fibres generally improve strength and modulus under tensile and flexural stress, lower the tendency to creep, improve heat resistance, and often also raise impact and notched impact resistance.

In practice, between 25-40% by weight of short glass fibre roughly doubles tensile strength but considerably lowers elongation. As the stress-strain diagram opposite shows, reinforced plastics have a brittle failure mode, without a pronounced strain limit.

For this reason, the tensile stress at yield of 60-90MPa of unreinforced engineering plastics may be compared with the tensile stress at break of 120-200MPa of reinforced plastics.

Addition of reinforcing materials, such as glass fibre, can raise stiffness to 20,000N/mm2. Amorphous polystyrene (PS) has an elongation at break of about 3%, whereas that of semi-crystalline polyethylene
(PE) can be as high as 1,000%. Typical values for impact resistance are in the 1–130 kJ/m2 range.

- Dupont

and glass fiber (which does stretch/flex) should not flex or deform is simply incorrect. It depends on the type of deformation and overall stress applied to the FRP; as to whether it has seen a load stress that has caused failure of the fibers.

You can sit and postulate for a long time. The OP asked whether there was any cause for concern about a dent in the bottom of a fibreglass boat. Yes there is. The dent may go away. The hull may still be strong enough that it ill never affect the performance or safety of the craft.

Or it might not.

Fibreglass is not an elastic material.

Best err on the side of caution.