They are mostly nocturnal, during the heat of the day they burrow just under the soil or hide in moist dark places. On cloudy and wet days they stay out later. The best times to find them are early in the morning while the dew is still on the leaves and just after dusk with a flashlight. When there are a lot of them, they will cover a sidewalk.
During the daytime, they hide under pots, in tall weeds in moist places.
I often find them eating the roots of my potted plants. I have to put screens in the drain holes to try to keep them out and pull the plants out of the pots once in a while and check the roots.
For every slug and snail you see, assume there are 20 you don't. If you see them out in the daytime, there are a whole lot more than you think, if they are foraging in the day.
They have been around since the Cambrian period, 500 million years, and like roaches will probably still be around long after we are gone. P.S. modern humans have only been around for a measly 200,000 years. Slugs and snails can potentially live for many years, since they are hermaphrodites everyone can lay eggs. They lay about 30 eggs at a time and eggs will hatch in about three weeks but eggs can be viable up to two years. Do the math, there numbers grow geometrically. That is why getting rid of them is pretty hard unless you can get rid of all your plants and you don't water a thing, then they will migrate off to your neighbors house instead. Getting rid of them, would take a lot of time and sluggo, control is probably the best you can hope for.
Frogs, toads, chickens, some songbirds, newts, and lizards eat slugs and snails. There are also cannibal snails that eat them as well.
Unfortunately, Hawaii has problems with introduced species. The mongoose is an infamous one. The cannibal snail was a deliberate introduction to control the other introduced African snail, however, the cannibal snail took a liking to a native snail and drove them to extinction. Considering how hard the Dept of Ag makes it to bring anything into the State, and their slow and sometimes stupid response time to invasive species, they still haven't figured out that bringing in other aliens doesn't guarantee they won't become problems themselves. They are too under staffed to inspect everything coming in and they don't have a lobby to get needed legislation to help them protect the aina.
Happy gardening in Hawaii. Gardens are where people grow.