Alder wood is the classical choice. Some of the milder woods, such as pecan, work good too. Avoid the more intense woods, such as mesquite, unless you want a super-smoky character.
The last yellow fin tuna I did was on Pecan, at 150 for 2 hours, then finished with hickory, at 200 until an internal temperature of 145 was reached. I had a sweet soy glaze I wanted to "set" at a higher temperature. The tuna finished with a beautiful reddish-bronze color, with a savory sweet smoky finish.