Not sure if I should carry coins for public transport in Marshall Islands or if card payments are common. Advice?
In my experience, credit card payments are not a thing for public transport in the Marshall Islands. It's an all-cash system. Taxis are very common, and the drivers will expect payment in US dollars. You should have no problem using your card at a major hotel, but for taxis or local businesses, cash is the only way. I'd recommend using an ATM at a bank in Majuro to get US dollars and have it on you at all times.
I found public transport to be a very simple cash-only affair in the Marshall Islands. There’s a community bus that runs, but taxis are the most common way to get around. They are cheap, but the drivers will only accept cash, US dollars. My tip is to make sure you have some small bills on you. A $20 bill might be a problem for a driver who doesn't have change, so have some $1 and $5 bills ready.
My advice is to not even bother thinking about a card for public transport. On Majuro, the main public transport is shared taxis, and they all operate on a cash-for-service basis. The locals use cash, and you should too. It's just a lot simpler that way. I’ve found the ATMs at the Bank of Marshall Islands to be the most reliable for getting US dollars. It’s the only way to ensure you can get around the island without any issues.
For getting around in the Marshall Islands, cash is king. I was on Majuro, and every taxi I hailed expected payment in US dollars. The drivers don't have card readers, and there's no payment app for the rides. It’s a very straightforward system, you pay your fare directly to the driver with cash. The best thing is to get some smaller US dollar bills from an ATM, as it's the official currency here anyway, and it'll make paying for your rides a lot easier.