I know what you mean about the rain, I've had the same issue on both of my machines (one equipped with a GeForce Ti4200 and the other with onboard Intel 845G/GL chipset). Don't know what to suggest I'm afraid, I tried everything too!
Seeing as this is a Mario Golf - Graphics Issue thread, I may as well ask about this problem as well. Despite what the GameFAQ says about the Missing grid, I CAN get it to appear on the machine equipped with the Intel 845G/GL chipset, but not on the GeForce. Any one else have this problem? Following the advice about this game from other threads (Turn on Self Rendered Textures, Use low res updates), I can get the game running at almost full speed on the Intel 845 equipped machine. Trouble is - it's my machine at work!
Jabo's 1.5.1 plugin is the best for this game, as none of the others can display all the graphics correctly. It's SOOOO almost there - anyone out there with some more suggestions?