As regards the sinking of your patrol boat, I take inspiration from this unfortunate event (it was resolved well only because the seabed was shallow), to say that all models can be made unsinkable with small precautions.
In theory, all ships could be unsinkable, but for real ones it's just theory, but for scale models it can actually be done.
Not for everyone, but almost.
Surely you already know it but I'm saying it for those who maybe don't know or are starting now:
1. Weigh the model. For example it weighs 4 kg.
2. 4 kg is equivalent to approximately 4 dm3 of fresh water (i.e. 4 litres).
3. I take cubes or parallelepipeds made of very light material that is not impregnable with water and easily cut, such as polystyrene, for example. I have to take a little more than 4 dm3. Let's say 4.5 dm3 to be safe. It's very simple because it's a 10cm side cube.
If you have light strips of material, that's fine as long as you are able to calculate their volume (B x h x L), so it's very easy with square (orthogonal) figures such as cubes or parallelepipeds.
4. By cutting this material in the way that best suits the model, you will have to insert it into all the empty places where it is possible to do so. I've seen a lot of your models with huge gaps.
I assure you that if a model weighing 4 kg has 4.5 dm3 of light material such as polystyrene inside it (well blocked) it will never sink.
For example, Caio has a piece of polystyrene (or similar) one meter long, 20 cm wide and 4 cm thick.
The total volume is 1000 x 200 x 40 mm = 8,000,000 mm3 = 8,000 cm3 = 8 dm3.
Therefore Caio will only need to use a little more than half (4 dm3) of this piece of polysitorol (or similar) to cut into many small parts to insert and secure well inside the model. The aesthetics will not be affected because everything is hidden.
I say just over half because polystyrene (or similar), however light, has its own weight.
Attention. This system does not prevent damage to electronics due to hull flooding.