Most of the yachts are owned by corporations or the rich and there are no local yacht charters that I know. However many of these yachts ( 50 footers and above ) are for rental on daily basis but you just have to know who to contact. If it is in Langkawi, you can actually enquire from The Royal Yacht Club. As far as Penang is concerned I know there are such yachts as I always see them chartered for fishing near Pulau Perak. I know of one known as 'White Elephant' which is moored in Penang. Maybe you can enquire from the Batu Maung slipway.

As for reasonable, I don't know what your 'reasonable' value is but I do know a 3D2N fishing charter can go to 8K in rental. For fishing wise, it is not much as 10 anglers will split the rental. As for your purpose, be prepared for about maybe 4K for 12 hours. There are also cheaper wooden yachts that is available in Lumut Yacht Club. Bonton in Langakwi also has their own private yacht which can be rented. The best bet in Penang is to enquire from Batu Maung Slipway now known as  ;

Main Office
Limbongan Batu Maung Sdn. Bhd.
204-b, Block K, MK 12, Batu Maung, 11960 Penang, Malaysia.

Tel : 604-6262628(Hunting Line)
Fax : 604-6262688
Email : yachts@lbmsb.com / lbmsb@pd.jaring.my

Although they operate mainly on repairs, production and storage, they know most of the yacht owners and so they are in the position of recommending the ones that are available for charter. good Luck!