The reason he got sleepy is cause he was high and very chill so he got tired cause he was not very active so next time do something funny and giggle your asses off lol
could be because in hot weather ur blood pressure naturally goes higher cuz its hotter, specially if ur not used to it or a native. That plus the fact that weed does the same to you is x2 so it can get pretty tirying just staying awake as ur blood is being pumped really hard... though thats just wat i think, no scientific backup on me xP
maybe he is so freakin high, the beach has nothin to do with it, and i love to smoke with friends on the beach and trippin on the waves try it its pretty cool, laugh your heart out man..........