Today, I hydromodded my Casio W-217H using basic, pure mineral oil from the grocery store. It was a bit of a pain to get it right and took several attempts, but it turned out okay, but I wanted to share a tip:
Don't use mineral oil (at least for the W-217H). It seems like oil comes out a bit when I press buttons. This led to a bubble. It might calm down, and I don't mind the bubble (creates a frutiger aero look), but I wanted to share this tip. Maybe silicone oil is better for this watch. The advantage is that mineral oil is way cheaper, but it should be noted that this mod is irreversible. You cannot take the oil out because that would require soap and water, so you need to do it right the first time. You can probably switch to silicone oil if mineral doesn't work out for you, but you cannot remove any oil once you start this.
Also, don't screw too hard. This watch is cheap so the shell is plastic and the screws chew them. I have one screw that cannot be fastened (keeps turning) so I am a little nervous about it coming undone. I tested the water resistance of the watch and it seems fine.
I won't bother showing the improvements in clarity because you can see this online, but I will say it's worth it, especially for seeing the display under water. The extra peace of mind for having this watch in water is nice too.
Here's a fun photo from an old movie my wife showed me :)
