Sasha said...
In fact you can use cheap normal glasses (with plastic lenses) but make sure ear bits are straight > easier to affix a band to prevent it from falling off the head.
Good luck.
I went down this road - people suggested $10 safety glasses from bunnings for example, and normal tinted sunnies.
I found that cheaper standard glasses are great if you never fall off, and one day I caught an edge, and did a faceplant.
On cheap sunnies, the lens can easily pop out on impact and almost take your eyes with them. Try squirting your sunnies with a high pressure hose on the back lawn and if the lens pop out, imagine your eyes behind them.
Also, the nose piece, which was designed to simply rest the sunnies on your nose as you walk around, is all of a sudden asked to take a huge amount of load. Check the shape and material of the nose piece - how will your nose handle that getting forced into it?
Any purpose built water sport sunnies feature a rubber nose piece that disperse the load on impact, have lenses designed to stay intact under load, lens are made of impact resistant material, and they are already equiped with straps.