Luffy generally just says whatever it is that he thinks but sometimes he just stares for a while and doesn’t say anything at all. So, I think when he wants to say something to Nami, he’ll immediately get her attention but then realize he doesn’t really know what to say?? So he just kind of stares while he’s figuring out the right words; but for Nami, all this is doing is just sending her “my captain is an idiot” signals.
But just when she’s about to walk away Luffy’s just “Your hair! It– looks like oranges. Tangerines? It’s orange. Like, really orange… I don’t know! I like it!!”
And that’s basically how he tells her that he thinks she’s pretty.