Does boat polyester resin works well for insect/flower encapsulating?
Disclaimer: I have no first-hand experience with industrial resins (like boat resin), but I used craft resin (like crystal epoxy resin) several times.

There are 2 aspects to keep in mind when choosing your resin:


The properties of the resin
Your own health


Material properties

For pouring and encasing objects in resin, you usually want a crystal clear resin with a low viscosity (very thin), to avoid air bubbles in the cast. To cover a big object like a boat in resin as a protective layer, the color is less important, but you want a thicker resin so it doesn't drip off.

The problem with pouring thick, viscous resins is that it's almost impossible to remove all the air bubbles without a vacuum or pressure chamber. This leaves you with a cloudy result (because of micro air bubbles) or with very annoying bubbles in tight spaces (like the petals of a flower). It can also displace light objects while being poured on them.

The yellowish color usually intensifies over time. You end up with something looking like amber. If that's what you're looking for, go for it, but it makes things harder to see inside.

Lastly, every kind of resin has a minimal and maximal pouring thickness. If you pour less, it takes forever to cure or doesn't cure at all. If you pour more, the material heats up to a boil and bubbles away (because of the chemical reaction). Many resins have a minimal thickness of 0, but some industrial resins need several millimeters thickness and your cast may be too small for it. Have a look at the packaging to get more information.

Health Considerations

Most polyester resins stink to hell and you really shouldn't inhale the fumes. The packages usually instruct you to only use it in very well ventilated areas and additionally wear a mask with a filter (not the simple dust mask but a real filter mask). Don't forget goggles and gloves*.

Epoxy resin doesn't stink much at all, but you shouldn't directly inhale the fumes either. An open window, goggles and gloves* are sufficient.

Don't forego the gloves, resins can cause allergic reactions that can manifest several hours after skin contact.

*: Depending on the resin you use, you need gloves of different materials. I use nitrile gloves for epoxy resin, but I don't know for sure if they're safe for polyester resin. 

If gloves are categorized as "safe" for certain chemicals, they resist those chemicals for a minimum of 8 hours. You should always use fresh gloves because after those 8 hours chemicals permeate the gloves.