Maratha generally does not build a lot of schools and, when run by the AI, generally does not conquer Mysore. This means that they can fall well behind on the tech tree (and seem to fall behind in everything but artillery). Do NOT trade techs with them beyond the minimum needed to secure trade early on. This will not slow them at first because Mughal is so inept againt the AI Maratha with its all raid approach that it will always come up a cropper. The earliest I have seen the Mughals go in 1.5 was in 1718 -- though 1728-35 is more common. You can tech and fund Mughal all you want but they will give you squat in return and it can only delay the inevitable by a few years.

Annoyingly (at least on H/H), Maratha seems to be set to attack you unless you're playing Prussia, Austria, or Poland. Those three may well be targeted also, but are usually so embroiled in the Central Euro mess that you can't develop overseas or other Maratha-accessible territory. Otherwise, they will target (and I've seen them go for) Iceland, Newfoundland, The Guyanas, The Leewards, Hispaniola, Ireland, and Morocco by sea and of course Meso or Dag/Geo if you're fighting with one of the Eastern Euro powers. They will pick their target based on what will force you to fight a three-way war at the greatest disadvantage.

Their first fleets will fall readily to a small (2-4) squadron of relatively powerful hitters (galleons, 5ths and up) and that is the best way to deal with them. If you see a Marathese fleet heading toward one of your sea territories, do not hesitate. They will not continue trading past the end of your move and they will attack regardless of your relationsh with them (alliance, etc. are irrelevant, they MUST hammer you). Just sink them.

The same method can protect your sea-access territories later on, but you will need a largish squadron 6-10 3rd rate and better ships. Late Maratha spams 2nd rates and will often sail with a half dozen PLUS other ships. This will necessitate a squadron in each area you have territory to protect (NA, Carib, Channel, Morocco) and is expensive. If you share a land border with them you have two viable choices: 1) a top quality full stack guarding a "must have" capital that they will not bypass (e.g. Esfahan) or 2) Conquer all of India. Half measures between the two will just get expensive as they can produce a snot-load of troops. It is possible to win 5 battles (their troops are no better and often much weaker) and lose the territory to their 6th stack.