I've flown in once on a 30 day entry and extended it for 30 days, then went by boat and did the same (4 months on  2 entries).Can I do it a third time - what are the risks of not getting back in? 

You could do another border entry (most likely)  but flying in could cause you a problem. You could get a setv. Depends how long you wish to stay

Edited July 31, 20196 yr by DrJack54

Entries by land or sea are limited to two per year. So you should be okay doing one more.