The disadvantage on geography makes Martell impossible to win by occupying 6 cities or strongholds. Tyrell can attack your city (Starfall) via sea transportation at any time. However, you have to march a long way if you want to attack Tyrell. Though you may take control of Storm End, I don't think you can easily defeat Baratheon in ShipBreaker because the support from BlackWater Bay can never be raided.

Furthermore, the total combat strenth of Martell's house card is 15 only. ( Greyjoy is 19, Lannister, Stark and Baratheon is 18, Tyrell is 16) In addition, the others have some hourse cards with powerful text ability that Martell don't have. The most important thing is that Martell hasn't a house card which can ignore opponent's text ability to against Tyrell and Baratheon.

Any tactics or ideas for Martell?

I've yet to play the Martells but I gathered they work best with ships. If I recall correctly (looong time since I played) there's a bay that connects to most of the Martell land so if you put 3 ships there and make them support you'll have a good defence. Might be the same if you build a fort at a bottle-neck. Based on the books they survived by being defensive and wearing others down. If Tyrell and Baratheon have no other qworries, then yeah, Martell's screwed. But once they have bigger problems or spread themselves thin you should be able to make your move. And if you can trap Baratheon's ships b/w yours and Stark's (or if B. sends most of their ships north to prevent Stark from building a navy) you could expand into the Stormlands.

Just to be sure, you only use them in a 6 player game right? Without the Greyjoys breathing down the Lannister's neck Tyrell won't be inclined to move toward the more profitable Riverlands and they will go for the Martells. Ports might make a difference too.