I have seen divers use a speadsheet. No you don’t need to buy MS, there is OpenOffice.org which is free.
There is a dive log right here on DB that you can print out, punch holes in and put in a binder with some dive logo that your LDS would be happy to sell you.
I’m still using Giant Stride that was written for Win95, it’s getting dicey on new platforms, I can no longer link pictures to the dive and it has no provisions for EANx but the remarks area goes on forever.