Engine Help please

I have a 2001 X-10. When the boat is in neutral, the engine runs and rev's up fine. But once I put it in gear, and hit the throttle, it backfires and will only go about 5mph at full throttle. Does anyone have any ideas of what it might be? I have had it in the shop and still am having the same problem. Any help or suggestions would be greatly appreciated!
Thanks!

It also backfires... But there is a new fuel filter, so I am stumped to what could be wrong, maybe something electrical?

[bigmac]

Timing may be off. Another possibility is plug wires out of normal firing order.

[corey]

Check the plugs and wires, my guess is one (or more) of them is bad.

[MCVOLS]

Had a similar problem after a tune up. Not as bad as yours sounds, but would backfire under a load. Idle was fine. Re-checked the 'new' plugs, and one was cracked. It got worse as we continued to run, I guess the crack got worse as it heated up. As mentioned, plugs/wires/timing sound like a logical starting spot.