And once on board, you never know how the crew of the suspicious vessel will react.
Even in benign conditions it's hard work - daytime temperatures in the Arabian Sea are still in the high 30s Celsius even in late September.
"Boarding a moving vessel in potentially rough seas requires a test of nerve and skill, especially when embarking and disembarking up a 15 foot ladder wearing body armour, life jacket, rifle and search equipment," explained Leading Seaman Kyle 'Bruce' Willis, normally one of Monmouth's weapon engineers, but a volunteer in the 'blue' boarding team.
It's down to the 'green' team of Royal Marines to secure a vessel and the 'blue' team of sailors to check paperwork, question the crew and conduct a thorough search for contraband, weapons and suspects if necessary.
Depending on the size of the vessel, it's painstaking and exhausting - but occasionally rewarding; a 60-hour search by the Black Duke's team earlier in the summer yielded a £65m secret stash of cocaine and heroin.
Both of the Black Duke's Pacific 24 seaboats were used for the first part of the exercise, as the teams conducted a few runs embarking and disembarking on the pilot ladder, it was time for them to practise the three-metre hook-on ladder (it hooks over the side of a deck).
During these runs, a dummy was used to act as a 'man overboard', simulating a member of the boarding team who had slipped off the ladder and ended up in the ocean.
"However simple climbing a ladder looks, it's made significantly more difficult when the vessel is moving at speed coupled with a choppy sea-state," said Marine Connor Aldrich.
"It's good to get all our drills practised and fully squared-away so when the operational boarding’s are required, the whole team are on the same page, ensuring a smooth and safe procedure."
With the Royal Marines content with their runs, it was the turn of the Royal Navy boarding team to practise their embarkation techniques.
"It was a good chance for us to work alongside the Royal Marines and to keep our boarding techniques slick and current," said Kyle.
Most board and search operations involve the two sea boats carrying sailors and marines, with the helicopter overhead observing, ready to intervene if something goes amiss.
In some cases, stealth and speed are required to secure a vessel before a crew are aware what has hit them - which requires 'rapid roping' from the ship's Wildcat helicopter, callsign Black Jack.
After a 'simple' practice on to the flight deck in everyday uniform, the commandos donned full boarding kit "which adds some serious weight", said Marine Oli Staite.
"You can feel the increase of speed on the way down the rope.
"Our final drill was fast roping on to the fo'c'sle of the ship, whilst it was underway at approximately ten knots to simulate the confined space and speeds that which we may face operationally."
With both teams happy with the results of the day's combined training, Monmouth has resumed her patrol of the Indian Ocean as part of the international task force committed to stopping terrorism/piracy/smuggling east of Suez.
"Conducting a regular drumbeat of boarding training keeps the team at the highest level of readiness to conduct interdiction operations," said Commander Ian Feasey, the frigate's Commanding Officer.
"It ensures the Black Duke can respond to protect UK interests and prevent illegal activities on the High Seas."
