Wood, Any wood is just that, it is inherently prone to warping,cracking, and waning. The straighter the grain the less prone it is to warping. I have gotten to where I can look at a board and tell if it is going to go south on me or not. To be honest the first dozen or so guns that I built were made of heart pine that I had recovered from a remodeling job in Williamsburg, Va. As far as I know the guns are still being used to slay huge catfish in the lakes and rivers of Arkansas. They were built of a single board of heart pine. But, those boards also were cut over a hundred years ago and had already waned or twisted all that they were going to, given the little that I trimmed off of them to make a gun. Wood is a strange enitity indeed, because the more you cut from a board the more that it generally wants to grow back to its general form(tubular). That is why most people lam. thier guns. because we as people are impatient, and it will take forver to find that perfect board that once cut does not wane, or warp. When you laminate two or more pieces together, you have different forces working against each other and kinda of equalizes the two sides. The reason that Collins says that, is because the price of Teak is so high and laminating is the best method to make use of the resources available. A speargun can be made either way, but the best and most economical method to use a lamination method.
Btw, It has been my experience that almost ANY wood can make an acceptable speargun. It all depends on the properties that you want in a speargun. Teak can last for years without proper care so as rubbing in some oil every couple of years. That is not to say that you should mistreat a teak gun by letting it go, and let the grain of the wood rise, this will affect your shots and your gun will be harder to take care of after that. There are many different woods out there that make great speargun woods such as: Jatoba, Jarrah, Bocote, and many others. The gun I use everyday is made of Ash, and is over 9 years old and has probably killed more fish than has ever been posted on this forum. It has been 60' deep in the Arkansas river killing 80 lb flathead catfish and in the gulf shooting snappers from 20' away. The gun has bounced around the granite rocks that face Murrey lock & Dam in Little Rock and been dropped from a boat over some sunken tanks in the Panhandle of Florida. I have never had to refinsih it or even replace the trigger. It has had the same Sea hornet trigger since I built it, the trigger has never failed even though I have had two 3/4" bands on it before. I wouldn't want to put that much pressure on it everyday, usually I use just one 5/8" 24"(soaked) band stretching the whole 48" to the first notch.
So, don't close your mind to different woods. They can be rewarding!